Phase 3 testing fixes: Fastify 5, SSE→WebSocket+FCM, enums, security, session lifecycle

- Upgrade Fastify 4→5 with all plugins (@fastify/websocket 11, cors 11, sensible 6)
- Migrate all SSE endpoints to WebSocket + FCM push (mitra chat requests, customer pairing status)
- Add flutter_local_notifications for foreground push notifications with sound
- Add splash screen to both apps (hide auth loading flash)
- Introduce constants/enums across entire codebase (no raw string literals)
- Move price tiers from hardcoded array to app_config DB (data-driven, includes 1-min test tier)
- Add session ownership validation on all shared chat routes
- Add ownership checks on endSession, respondToExtension, requestExtension
- Fix session timer: auto-complete expired/stale sessions on server restart
- Add 5-min grace period for abandoned closing sessions
- Fix extension flow: proper session_resumed handling, clearExtensionRequest, closure grace timer cleanup
- Fix chat screens: ConnectChat in initState, session status check on connect
- Fix customer expired view: 5-min countdown, closure state priority over expired state
- Fix mitra extension UI: loading spinner, disable buttons, handle EXTENSION_RESOLVED error
- Fix GoRouter navigation consistency (no more Navigator.pushNamed)
- Fix goodbye view keyboard overflow (SingleChildScrollView)
- Add active session card on customer home screen with refresh on navigate back
- Fix PricingBottomSheet extension mode (RequestExtension instead of new pairing)
- Send session_resumed to both parties on extension accept

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/package.json

@@ -11,11 +11,11 @@
     "db:seed": "node src/db/seed.js"
   },
   "dependencies": {
-    "@fastify/cors": "^9.0.1",
-    "@fastify/sensible": "^5.6.0",
-    "@fastify/websocket": "^11.2.0",
+    "@fastify/cors": "^11.0.0",
+    "@fastify/sensible": "^6.0.0",
+    "@fastify/websocket": "^11.0.0",
     "dotenv": "^16.4.5",
-    "fastify": "^4.28.1",
+    "fastify": "^5.0.0",
     "firebase-admin": "^12.2.0",
     "ioredis": "^5.4.1",
     "pg": "^8.12.0",