R3: dashboards, docs and tracker bake-back

- Edge/UX dashboard: aggregate request-rate vs rejection-rate panel
  (gateway_rate_limited_total by class; no per-user labels).
- ARCHITECTURE §2/§11/§12/§13: body cap + explicit h2c sizing, the rate-limit
  observability pipeline and auto-flag policy, the admin-limiter note (and the
  caddy-path gap), the landing container topology; fixed the stale 120/min
  per-user figure.
- FUNCTIONAL (+_ru): the Throttled view and the reversible high-rate flag.
- gateway/backend/deploy READMEs, TESTING.md, root CLAUDE.md updated.
- PRERELEASE.md: R3 interview decisions + implementation refinements logged;
  tracker R3 -> done (this PR implements it; CI gates the merge).

.gitignore

@@ -16,3 +16,6 @@
 # Local, unstaged env overrides
 **/.env.local
 **/.env.*.local
+
+# Claude Code harness runtime artifacts
+.claude/scheduled_tasks.lock