chore(ci): align Go to 1.23.x, add staticcheck/govulncheck/gitleaks gates

.github/workflows/ci.yml:
- Go version: 1.22 -> 1.23.4 (matches go.mod's 'go 1.23.0' declaration).
- Split into four jobs with explicit names:
    * test-backend: go vet + go build + go test
    * scan-backend: staticcheck + govulncheck (installed from pinned tags)
    * test-frontend: npm ci + eslint + tsc --noEmit + next build
    * gitleaks: full-history secret scan on every PR
- Branches triggered: master + main + develop (master is the repo
  default; the previous workflow only triggered on main/develop and
  would never have run on the repo's actual PRs).
- actions/checkout@v4, actions/setup-go@v5, actions/setup-node@v4.
- Concurrency group cancels stale runs on the same ref.
- Node and Go caches enabled for faster CI.

.gitleaks.toml (new):
- Extends gitleaks defaults.
- Custom rule 'explorer-legacy-db-password-L@ker' keeps the historical
  password pattern L@kers?\$?2010 wedged in the detection set even
  after rotation, so any re-introduction (via copy-paste from old
  branches, stale docs, etc.) fails CI.
- Allowlists docs/SECURITY.md and CHANGELOG.md where the string is
  cited in rotation context.

backend/staticcheck.conf (new):
- Enables the full SA* correctness set.
- Temporarily disables ST1000/1003/1005/1020/1021/1022, U1000, S1016,
  S1031. These are stylistic/cosmetic checks; the project has a long
  tail of pre-existing hits there that would bloat every PR. Each is
  commented so the disable can be reverted in a dedicated cleanup.

Legit correctness issues surfaced by staticcheck and fixed in this PR:
- backend/analytics/token_distribution.go: 'best-effort MV refresh'
  block no longer dereferences a shadowed 'err'; scope-tight 'if err :='
  used for the subsequent QueryRow.
- backend/api/rest/middleware.go: compressionMiddleware() was parsing
  Accept-Encoding and doing nothing with it. Now it's a literal
  pass-through with a TODO comment pointing at gorilla/handlers.
- backend/api/rest/mission_control.go: shadowed 'err' from
  json.Unmarshal was assigned to an ignored outer binding via
  fmt.Errorf; replaced with a scoped 'if uerr :=' that lets the RPC
  fallback run as intended.
- backend/indexer/traces/tracer.go: best-effort CREATE TABLE no longer
  discards the error implicitly.
- backend/indexer/track2/block_indexer.go: 'latestBlock - uint64(i) >= 0'
  was a tautology on uint64. Replaced with an explicit
  'if uint64(i) > latestBlock { break }' guard so operators running
  count=1000 against a shallow chain don't underflow.
- backend/tracing/tracer.go: introduces a local ctxKey type and two
  constants so WithValue calls stop tripping SA1029.

Verification:
- go build ./... clean.
- go vet ./... clean.
- go test ./... all existing tests PASS.
- staticcheck ./... clean except for the SA1029 hits in
  api/middleware/auth.go and api/track4/operator_scripts_test.go,
  which are resolved by PR #4 once it merges to master.

Advances completion criterion 4 (CI in good health).

backend/api/rest/middleware.go

@@ -41,14 +41,11 @@ func (s *Server) loggingMiddleware(next http.Handler) http.Handler {
 	})
 }
 
-// compressionMiddleware adds gzip compression (simplified - use gorilla/handlers in production)
+// compressionMiddleware is a pass-through today; it exists so that the
+// routing stack can be composed without conditionals while we evaluate the
+// right compression approach (likely gorilla/handlers.CompressHandler in a
+// follow-up). Accept-Encoding parsing belongs in the real implementation;
+// doing it here without acting on it just adds overhead.
 func (s *Server) compressionMiddleware(next http.Handler) http.Handler {
-	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
-		// Check if client accepts gzip
-		if r.Header.Get("Accept-Encoding") != "" {
-			// In production, use gorilla/handlers.CompressHandler
-			// For now, just pass through
-		}
-		next.ServeHTTP(w, r)
-	})
+	return next
 }

backend/api/rest/mission_control.go

@@ -475,8 +475,12 @@ func (s *Server) HandleMissionControlBridgeTrace(w http.ResponseWriter, r *http.
 	body, statusCode, err := fetchBlockscoutTransaction(r.Context(), tx)
 	if err == nil && statusCode == http.StatusOK {
 		var txDoc map[string]interface{}
-		if err := json.Unmarshal(body, &txDoc); err != nil {
-			err = fmt.Errorf("invalid blockscout JSON")
+		if uerr := json.Unmarshal(body, &txDoc); uerr != nil {
+			// Fall through to the RPC fallback below. The HTTP fetch
+			// succeeded but the body wasn't valid JSON; letting the code
+			// continue means we still get addresses from RPC instead of
+			// failing the whole request.
+			_ = uerr
 		} else {
 			fromAddr = extractEthAddress(txDoc["from"])
 			toAddr = extractEthAddress(txDoc["to"])