fix(auth): typed context keys and real sentinel errors

backend/api/middleware/context.go (new):
- Introduces an unexported ctxKey type and three constants
  (ctxKeyUserAddress, ctxKeyUserTrack, ctxKeyAuthenticated) that
  replace the bare string keys 'user_address', 'user_track', and
  'authenticated'. Bare strings trigger go vet's SA1029 and collide
  with keys from any other package that happens to share the name.
- Helpers: ContextWithAuth, UserAddress, UserTrack, IsAuthenticated.
- Sentinel: ErrMissingAuthorization replaces the misuse of
  http.ErrMissingFile as an auth-missing signal. (http.ErrMissingFile
  belongs to multipart form parsing and was semantically wrong.)

backend/api/middleware/auth.go:
- RequireAuth, OptionalAuth, RequireTrack now all read/write via the
  helpers; no more string literals for context keys in this file.
- extractAuth returns ErrMissingAuthorization instead of
  http.ErrMissingFile.
- Dropped now-unused 'context' import.

backend/api/track4/operator_scripts.go, backend/api/track4/endpoints.go,
backend/api/rest/features.go:
- Read user address / track via middleware.UserAddress() and
  middleware.UserTrack() instead of a raw context lookup with a bare
  string key.
- Import 'github.com/explorer/backend/api/middleware'.

backend/api/track4/operator_scripts_test.go:
- Four test fixtures updated to seed the request context through
  middleware.ContextWithAuth (track 4, authenticated) instead of
  context.WithValue with a bare 'user_address' string. This is the
  load-bearing change that proves typed keys are required: a bare
  string key no longer wakes up the middleware helpers.

backend/api/middleware/context_test.go (new):
- Round-trip test for ContextWithAuth + UserAddress + UserTrack +
  IsAuthenticated.
- Defaults: UserTrack=1, UserAddress="", IsAuthenticated=false on a
  bare context.
- TestContextKeyIsolation: an outside caller that inserts
  'user_address' as a bare string key must NOT be visible to
  UserAddress; proves the type discipline.
- ErrMissingAuthorization sentinel smoke test.

Verification:
- go build ./... clean.
- go vet ./... clean (removes SA1029 on the old bare keys).
- go test ./api/middleware/... ./api/track4/... ./api/rest/... PASS.

Advances completion criterion 3 (Auth correctness).

backend/api/middleware/auth.go

@@ -1,7 +1,6 @@
 package middleware
 
 import (
-	"context"
 	"fmt"
 	"net/http"
 	"strings"
@@ -31,11 +30,7 @@ func (m *AuthMiddleware) RequireAuth(next http.Handler) http.Handler {
 			return
 		}
 
-		// Add user context
-		ctx := context.WithValue(r.Context(), "user_address", address)
-		ctx = context.WithValue(ctx, "user_track", track)
-		ctx = context.WithValue(ctx, "authenticated", true)
-
+		ctx := ContextWithAuth(r.Context(), address, track, true)
 		next.ServeHTTP(w, r.WithContext(ctx))
 	})
 }
@@ -44,11 +39,7 @@ func (m *AuthMiddleware) RequireAuth(next http.Handler) http.Handler {
 func (m *AuthMiddleware) RequireTrack(requiredTrack int) func(http.Handler) http.Handler {
 	return func(next http.Handler) http.Handler {
 		return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
-			// Extract track from context (set by RequireAuth or OptionalAuth)
-			track, ok := r.Context().Value("user_track").(int)
-			if !ok {
-				track = 1 // Default to Track 1 (public)
-			}
+			track := UserTrack(r.Context())
 
 			if !featureflags.HasAccess(track, requiredTrack) {
 				writeForbidden(w, requiredTrack)
@@ -65,40 +56,33 @@ func (m *AuthMiddleware) OptionalAuth(next http.Handler) http.Handler {
 	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
 		address, track, err := m.extractAuth(r)
 		if err != nil {
-			// No auth provided, default to Track 1 (public)
-			ctx := context.WithValue(r.Context(), "user_address", "")
-			ctx = context.WithValue(ctx, "user_track", 1)
-			ctx = context.WithValue(ctx, "authenticated", false)
+			// No auth provided (or auth failed) — fall back to Track 1.
+			ctx := ContextWithAuth(r.Context(), "", defaultTrackLevel, false)
 			next.ServeHTTP(w, r.WithContext(ctx))
 			return
 		}
 
-		// Auth provided, add user context
-		ctx := context.WithValue(r.Context(), "user_address", address)
-		ctx = context.WithValue(ctx, "user_track", track)
-		ctx = context.WithValue(ctx, "authenticated", true)
-
+		ctx := ContextWithAuth(r.Context(), address, track, true)
 		next.ServeHTTP(w, r.WithContext(ctx))
 	})
 }
 
-// extractAuth extracts authentication information from request
+// extractAuth extracts authentication information from the request.
+// Returns ErrMissingAuthorization when no usable Bearer token is present;
+// otherwise returns the error from JWT validation.
 func (m *AuthMiddleware) extractAuth(r *http.Request) (string, int, error) {
-	// Get Authorization header
 	authHeader := r.Header.Get("Authorization")
 	if authHeader == "" {
-		return "", 0, http.ErrMissingFile
+		return "", 0, ErrMissingAuthorization
 	}
 
-	// Check for Bearer token
 	parts := strings.Split(authHeader, " ")
 	if len(parts) != 2 || parts[0] != "Bearer" {
-		return "", 0, http.ErrMissingFile
+		return "", 0, ErrMissingAuthorization
 	}
 
 	token := parts[1]
 
-	// Validate JWT token
 	address, track, err := m.walletAuth.ValidateJWT(token)
 	if err != nil {
 		return "", 0, err

backend/api/middleware/context.go

@@ -0,0 +1,60 @@
+package middleware
+
+import (
+	"context"
+	"errors"
+)
+
+// ctxKey is an unexported type for request-scoped authentication values.
+// Using a distinct type (rather than a bare string) keeps our keys out of
+// collision range for any other package that also calls context.WithValue,
+// and silences go vet's SA1029.
+type ctxKey string
+
+const (
+	ctxKeyUserAddress   ctxKey = "user_address"
+	ctxKeyUserTrack     ctxKey = "user_track"
+	ctxKeyAuthenticated ctxKey = "authenticated"
+)
+
+// Default track level applied to unauthenticated requests (Track 1 = public).
+const defaultTrackLevel = 1
+
+// ErrMissingAuthorization is returned by extractAuth when no usable
+// Authorization header is present on the request. Callers should treat this
+// as "no auth supplied" rather than a hard failure for optional-auth routes.
+var ErrMissingAuthorization = errors.New("middleware: authorization header missing or malformed")
+
+// ContextWithAuth returns a child context carrying the supplied
+// authentication state. It is the single place in the package that writes
+// the auth context keys.
+func ContextWithAuth(parent context.Context, address string, track int, authenticated bool) context.Context {
+	ctx := context.WithValue(parent, ctxKeyUserAddress, address)
+	ctx = context.WithValue(ctx, ctxKeyUserTrack, track)
+	ctx = context.WithValue(ctx, ctxKeyAuthenticated, authenticated)
+	return ctx
+}
+
+// UserAddress returns the authenticated wallet address stored on ctx, or
+// "" if the context is not authenticated.
+func UserAddress(ctx context.Context) string {
+	addr, _ := ctx.Value(ctxKeyUserAddress).(string)
+	return addr
+}
+
+// UserTrack returns the access tier recorded on ctx. If no track was set
+// (e.g. the request bypassed all auth middleware) the caller receives
+// Track 1 (public) so route-level checks can still make a decision.
+func UserTrack(ctx context.Context) int {
+	if track, ok := ctx.Value(ctxKeyUserTrack).(int); ok {
+		return track
+	}
+	return defaultTrackLevel
+}
+
+// IsAuthenticated reports whether the current request carried a valid auth
+// token that was successfully parsed by the middleware.
+func IsAuthenticated(ctx context.Context) bool {
+	ok, _ := ctx.Value(ctxKeyAuthenticated).(bool)
+	return ok
+}

backend/api/middleware/context_test.go

@@ -0,0 +1,62 @@
+package middleware
+
+import (
+	"context"
+	"errors"
+	"testing"
+)
+
+func TestContextWithAuthRoundTrip(t *testing.T) {
+	ctx := ContextWithAuth(context.Background(), "0xabc", 4, true)
+
+	if got := UserAddress(ctx); got != "0xabc" {
+		t.Fatalf("UserAddress() = %q, want %q", got, "0xabc")
+	}
+	if got := UserTrack(ctx); got != 4 {
+		t.Fatalf("UserTrack() = %d, want 4", got)
+	}
+	if !IsAuthenticated(ctx) {
+		t.Fatal("IsAuthenticated() = false, want true")
+	}
+}
+
+func TestUserTrackDefaultsToTrack1OnBareContext(t *testing.T) {
+	if got := UserTrack(context.Background()); got != defaultTrackLevel {
+		t.Fatalf("UserTrack(empty) = %d, want %d", got, defaultTrackLevel)
+	}
+}
+
+func TestUserAddressEmptyOnBareContext(t *testing.T) {
+	if got := UserAddress(context.Background()); got != "" {
+		t.Fatalf("UserAddress(empty) = %q, want empty", got)
+	}
+}
+
+func TestIsAuthenticatedFalseOnBareContext(t *testing.T) {
+	if IsAuthenticated(context.Background()) {
+		t.Fatal("IsAuthenticated(empty) = true, want false")
+	}
+}
+
+// TestContextKeyIsolation proves that the typed ctxKey values cannot be
+// shadowed by a caller using bare-string keys with the same spelling.
+// This is the specific class of bug fixed by this PR.
+func TestContextKeyIsolation(t *testing.T) {
+	ctx := context.WithValue(context.Background(), "user_address", "injected")
+	if got := UserAddress(ctx); got != "" {
+		t.Fatalf("expected empty address (bare string key must not collide), got %q", got)
+	}
+}
+
+func TestErrMissingAuthorizationIsSentinel(t *testing.T) {
+	if ErrMissingAuthorization == nil {
+		t.Fatal("ErrMissingAuthorization must not be nil")
+	}
+	wrapped := errors.New("wrapped: " + ErrMissingAuthorization.Error())
+	if errors.Is(wrapped, ErrMissingAuthorization) {
+		t.Fatal("string-wrapped error must not satisfy errors.Is (smoke check)")
+	}
+	if !errors.Is(ErrMissingAuthorization, ErrMissingAuthorization) {
+		t.Fatal("ErrMissingAuthorization must satisfy errors.Is against itself")
+	}
+}

backend/api/rest/features.go

@@ -4,6 +4,7 @@ import (
 	"encoding/json"
 	"net/http"
 
+	"github.com/explorer/backend/api/middleware"
 	"github.com/explorer/backend/featureflags"
 )
 
@@ -16,11 +17,8 @@ func (s *Server) handleFeatures(w http.ResponseWriter, r *http.Request) {
 	}
 
 	// Extract user track from context (set by auth middleware)
-	// Default to Track 1 (public) if not authenticated
-	userTrack := 1
-	if track, ok := r.Context().Value("user_track").(int); ok {
-		userTrack = track
-	}
+	// Default to Track 1 (public) if not authenticated (handled by helper).
+	userTrack := middleware.UserTrack(r.Context())
 
 	// Get enabled features for this track
 	enabledFeatures := featureflags.GetEnabledFeatures(userTrack)

backend/api/track4/endpoints.go

@@ -12,6 +12,7 @@ import (
 	"strings"
 	"time"
 
+	"github.com/explorer/backend/api/middleware"
 	"github.com/explorer/backend/auth"
 	"github.com/jackc/pgx/v5/pgxpool"
 )
@@ -185,7 +186,7 @@ func (s *Server) requireOperatorAccess(w http.ResponseWriter, r *http.Request) (
 		return "", "", false
 	}
 
-	operatorAddr, _ := r.Context().Value("user_address").(string)
+	operatorAddr := middleware.UserAddress(r.Context())
 	operatorAddr = strings.TrimSpace(operatorAddr)
 	if operatorAddr == "" {
 		writeError(w, http.StatusUnauthorized, "unauthorized", "Operator address required")

backend/api/track4/operator_scripts.go

@@ -13,6 +13,8 @@ import (
 	"path/filepath"
 	"strings"
 	"time"
+
+	"github.com/explorer/backend/api/middleware"
 )
 
 type runScriptRequest struct {
@@ -67,7 +69,7 @@ func (s *Server) HandleRunScript(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
-	operatorAddr, _ := r.Context().Value("user_address").(string)
+	operatorAddr := middleware.UserAddress(r.Context())
 	if operatorAddr == "" {
 		writeError(w, http.StatusUnauthorized, "unauthorized", "Operator address required")
 		return

backend/api/track4/operator_scripts_test.go

@@ -11,6 +11,7 @@ import (
 	"net/http"
 	"net/http/httptest"
 
+	"github.com/explorer/backend/api/middleware"
 	"github.com/stretchr/testify/require"
 )
 
@@ -45,7 +46,7 @@ func TestHandleRunScriptUsesForwardedClientIPAndRunsAllowlistedScript(t *testing
 
 	reqBody := []byte(`{"script":"echo.sh","args":["world"]}`)
 	req := httptest.NewRequest(http.MethodPost, "/api/v1/track4/operator/run-script", bytes.NewReader(reqBody))
-	req = req.WithContext(context.WithValue(req.Context(), "user_address", "0x4A666F96fC8764181194447A7dFdb7d471b301C8"))
+	req = req.WithContext(middleware.ContextWithAuth(req.Context(), "0x4A666F96fC8764181194447A7dFdb7d471b301C8", 4, true))
 	req.RemoteAddr = "10.0.0.10:8080"
 	req.Header.Set("X-Forwarded-For", "203.0.113.9, 10.0.0.10")
 	w := httptest.NewRecorder()
@@ -77,7 +78,7 @@ func TestHandleRunScriptRejectsNonAllowlistedScript(t *testing.T) {
 	s := &Server{roleMgr: &stubRoleManager{allowed: true}, chainID: 138}
 
 	req := httptest.NewRequest(http.MethodPost, "/api/v1/track4/operator/run-script", bytes.NewReader([]byte(`{"script":"blocked.sh"}`)))
-	req = req.WithContext(context.WithValue(req.Context(), "user_address", "0x4A666F96fC8764181194447A7dFdb7d471b301C8"))
+	req = req.WithContext(middleware.ContextWithAuth(req.Context(), "0x4A666F96fC8764181194447A7dFdb7d471b301C8", 4, true))
 	req.RemoteAddr = "127.0.0.1:9999"
 	w := httptest.NewRecorder()
 
@@ -100,7 +101,7 @@ func TestHandleRunScriptRejectsFilenameCollisionOutsideAllowlistedPath(t *testin
 	s := &Server{roleMgr: &stubRoleManager{allowed: true}, chainID: 138}
 
 	req := httptest.NewRequest(http.MethodPost, "/api/v1/track4/operator/run-script", bytes.NewReader([]byte(`{"script":"unsafe/backup.sh"}`)))
-	req = req.WithContext(context.WithValue(req.Context(), "user_address", "0x4A666F96fC8764181194447A7dFdb7d471b301C8"))
+	req = req.WithContext(middleware.ContextWithAuth(req.Context(), "0x4A666F96fC8764181194447A7dFdb7d471b301C8", 4, true))
 	req.RemoteAddr = "127.0.0.1:9999"
 	w := httptest.NewRecorder()
 
@@ -122,7 +123,7 @@ func TestHandleRunScriptTruncatesLargeOutput(t *testing.T) {
 	s := &Server{roleMgr: &stubRoleManager{allowed: true}, chainID: 138}
 
 	req := httptest.NewRequest(http.MethodPost, "/api/v1/track4/operator/run-script", bytes.NewReader([]byte(`{"script":"large.sh"}`)))
-	req = req.WithContext(context.WithValue(req.Context(), "user_address", "0x4A666F96fC8764181194447A7dFdb7d471b301C8"))
+	req = req.WithContext(middleware.ContextWithAuth(req.Context(), "0x4A666F96fC8764181194447A7dFdb7d471b301C8", 4, true))
 	req.RemoteAddr = "127.0.0.1:9999"
 	w := httptest.NewRecorder()