test(e2e): add make e2e-full target, full-stack Playwright spec, CI wiring, docs

Closes the 'e2e tests only hit production; no local full-stack harness'
finding from the review. The existing e2e suite
(scripts/e2e-explorer-frontend.spec.ts) runs against explorer.d-bis.org
and so can't validate a PR before it merges -- it's a production canary,
not a pre-merge gate.

This PR adds a parallel harness that stands the entire stack up locally
(postgres + elasticsearch + redis via docker-compose, backend API, and
a production build of the frontend) and runs a Playwright smoke spec
against it. It is wired into Make and into a dedicated CI workflow.

Changes:

scripts/e2e-full.sh (new, chmod +x):
  - docker compose -p explorer-e2e up -d postgres elasticsearch redis.
  - Waits for postgres readiness (pg_isready loop).
  - Runs database/migrations/migrate.go so schema + seeds including
    the new 0016_jwt_revocations table from PR #8 are applied.
  - Starts 'go run ./backend/api/rest' on :8080; waits for /healthz.
  - Builds + starts 'npm run start' on :3000; waits for a 200.
  - npx playwright install --with-deps chromium; runs the full-stack
    spec; tears down docker and kills the backend+frontend processes
    via an EXIT trap. E2E_KEEP_STACK=1 bypasses teardown for
    interactive debugging.
  - Generates an ephemeral JWT_SECRET per run so stale tokens don't
    bleed across runs (and the fail-fast check from PR #3 passes).
  - Provides a dev-safe CSP_HEADER default so PR #3's hardened
    production CSP check doesn't reject localhost connections.

scripts/e2e-full-stack.spec.ts (new):
  - Playwright spec that exercises public routes + a couple of
    backend endpoints. Takes a full-page screenshot of each route
    into test-results/screenshots/<route>.png so reviewers can
    eyeball the render from CI artefacts.
  - Covers: /healthz, /, /blocks, /transactions, /addresses, /tokens,
    /pools, /search, /wallet, /routes, /api/v1/access/products (YAML
    catalogue from PR #7), /api/v1/auth/nonce (SIWE kickoff).
  - Sticks to Track-1 (no wallet auth needed) so it can run in CI
    without provisioning a test wallet.

playwright.config.ts:
  - Broadened testMatch from a single filename to /e2e-.*\.spec\.ts/
    so the new spec is picked up alongside the existing production
    canary spec. fullyParallel, worker, timeout, reporter, and
    project configuration unchanged.

Makefile:
  - New 'e2e-full' target -> ./scripts/e2e-full.sh. Listed in 'help'.
  - test-e2e (production canary) left untouched.

.github/workflows/e2e-full.yml (new):
  - Dedicated workflow, NOT on every push/PR (the full stack takes
    minutes and requires docker). Triggers:
      * workflow_dispatch (manual)
      * PRs labelled run-e2e-full (opt-in for changes that touch
        migrations, auth, or routing)
      * nightly schedule (04:00 UTC)
  - Uses Go 1.23.x and Node 20 to match PR #5's pinning.
  - Uploads two artefacts on every run: e2e-screenshots
    (test-results/screenshots/) and playwright-report.

docs/TESTING.md (new):
  - Four-tier test pyramid: unit -> static analysis -> production
    canary -> full-stack Playwright.
  - Env var reference table for e2e-full.sh.
  - How to trigger the CI workflow.

Verification:
  bash -n scripts/e2e-full.sh                 clean
  The spec imports compile cleanly against the existing @playwright
  /test v1.40 declared in the root package.json; no new runtime
  dependencies are added.
  Existing scripts/e2e-explorer-frontend.spec.ts still matched by
  the broadened testMatch regex.

Advances completion criterion 7 (end-to-end coverage): 'make e2e-full
boots the real stack, Playwright runs against it, CI uploads
screenshots, a nightly job catches regressions that only show up
when all services are live.'

docs/TESTING.md

@@ -0,0 +1,86 @@
+# Testing
+
+The explorer has four test tiers. Run them in order of fidelity when
+debugging a regression.
+
+## 1. Unit / package tests
+
+Fast. Run on every PR.
+
+```bash
+# Backend
+cd backend && go test ./...
+
+# Frontend
+cd frontend && npm test          # lint + type-check
+cd frontend && npm run test:unit # vitest
+```
+
+## 2. Static analysis
+
+Blocking on CI since PR #5 (`chore(ci): align Go to 1.23.x, add
+staticcheck/govulncheck/gitleaks gates`).
+
+```bash
+cd backend && staticcheck ./...
+cd backend && govulncheck ./...
+git diff master... | gitleaks protect --staged --config ../.gitleaks.toml
+```
+
+## 3. Production-targeting Playwright
+
+Runs against `https://explorer.d-bis.org` (or the URL in `EXPLORER_URL`)
+and only checks public routes. Useful as a production canary; wired
+into the `test-e2e` Make target.
+
+```bash
+EXPLORER_URL=https://explorer.d-bis.org make test-e2e
+```
+
+## 4. Full-stack Playwright (`make e2e-full`)
+
+Spins up the entire stack locally — `postgres`, `elasticsearch`,
+`redis` via docker-compose, plus a local build of `backend/api/rest`
+and `frontend` — then runs the full-stack Playwright spec against it.
+
+```bash
+make e2e-full
+```
+
+What it does, in order:
+
+1. `docker compose -p explorer-e2e up -d postgres elasticsearch redis`
+2. Wait for Postgres readiness.
+3. Run `go run database/migrations/migrate.go` to apply schema +
+   seeds (including `0016_jwt_revocations` from PR #8).
+4. `go run ./backend/api/rest` on port `8080`.
+5. `npm ci && npm run build && npm run start` on port `3000`.
+6. `npx playwright test scripts/e2e-full-stack.spec.ts`.
+7. Tear everything down (unless `E2E_KEEP_STACK=1`).
+
+Screenshots of every route are written to
+`test-results/screenshots/<route>.png`.
+
+### Env vars
+
+| Var | Default | Purpose |
+|-----|---------|---------|
+| `EXPLORER_URL` | `http://localhost:3000` | Frontend base URL for the spec |
+| `EXPLORER_API_URL` | `http://localhost:8080` | Backend base URL |
+| `JWT_SECRET` | generated per-run | Required by backend fail-fast check (PR #3) |
+| `CSP_HEADER` | dev-safe default | Same |
+| `E2E_KEEP_STACK` | `0` | If `1`, leave the stack up after the run |
+| `E2E_SKIP_DOCKER` | `0` | If `1`, assume docker services already running |
+| `E2E_SCREENSHOT_DIR` | `test-results/screenshots` | Where to write PNGs |
+
+### CI integration
+
+`.github/workflows/e2e-full.yml` runs `make e2e-full` on:
+
+* **Manual** trigger (`workflow_dispatch`).
+* **PRs labelled `run-e2e-full`** — apply the label when a change
+  warrants full-stack validation (migrations, auth, routing changes).
+* **Nightly** at 04:00 UTC.
+
+Screenshots and the Playwright HTML report are uploaded as build
+artefacts.