feat(08-03): push notification backend — DB model, migration, API router, VAPID setup

- Add PushSubscription ORM model with unique(user_id, endpoint) constraint
- Add Alembic migration 012 for push_subscriptions table
- Add push router (subscribe, unsubscribe, send) in shared/api/push.py
- Mount push router in gateway/main.py
- Add pywebpush to gateway dependencies for server-side VAPID delivery
- Wire push trigger into WebSocket handler (fires when client disconnects mid-stream)
- Add VAPID keys to .env / .env.example
- Add push/install i18n keys in en/es/pt message files

packages/gateway/pyproject.toml

@@ -18,6 +18,7 @@ dependencies = [
     "httpx>=0.28.0",
     "redis>=5.0.0",
     "boto3>=1.35.0",
+    "pywebpush>=2.0.0",
 ]
 
 [tool.uv.sources]