feat(F-004): typed fail-fast config and feature flag module

- loadConfig: pure over env object, accumulates all problems, names var names only
- DATABASE_URL now required at startup; PORT/HOST/LOG_LEVEL/NODE_ENV/REDIS_URL defaulted
- flags module behind FeatureFlagProvider; unknown flags OFF; runtime setEnabled (no redeploy)
- buildApp decorates app.flags; server.ts fail-fast before app boot
- tests caught and fixed flag-store case-normalization bug before gates
- zero new dependencies; all gates approved; verify.sh green

work/runtime-status.json

@@ -6,6 +6,6 @@
   "state": "waiting",
   "next_agent": "leader",
   "waiting_for": "Seleccionar una feature pending y actualizar este estado",
-  "updated_at": "2026-08-14T20:13:28Z",
+  "updated_at": "2026-08-14T20:29:18Z",
   "timeline": []
 }