Phase 4: Web Push notifications (pywebpush + VAPID)

Backend: PushSubscription model/migration, VAPID config + `cli.py
generate-vapid-keys`, push_service.send_push_to_user (upsert-by-endpoint
subscribe/unsubscribe, auto-cleanup of expired 404/410 subscriptions),
/api/push/* router, and ConnectionManager now tracks connected user IDs
per room so chat.py can push only to offline members after broadcasting
to online ones.

Two test-infra bugs found and fixed along the way: send_push_to_user
takes the caller's AsyncSession and is awaited inline rather than fired
via asyncio.create_task with its own session (background tasks were
outliving the test event loop); and the ws_client fixture now uses
NullPool to eliminate a connection-pool checkout race that was failing
WS tests intermittently.

Frontend: service worker rebuilt with vite-plugin-pwa's injectManifest
strategy (custom src/sw.ts) so it can add push/notificationclick
handlers alongside the existing precaching and StaleWhileRevalidate
routes ported over from generateSW. New subscribe/unsubscribe flow
(lib/push.ts, api/push.ts) with a toggle in the account menu.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

backend/app/config.py

@@ -9,5 +9,12 @@ class Settings(BaseSettings):
     session_https_only: bool = True
     session_max_age_seconds: int = 60 * 60 * 24 * 14
 
+    # Optional: push notifications are skipped (logged, not an error) if
+    # unset, so existing deployments don't have to configure this to keep
+    # running. Generate a pair with `python -m app.cli generate-vapid-keys`.
+    vapid_public_key: str | None = None
+    vapid_private_key: str | None = None
+    vapid_subject: str = "mailto:admin@example.com"
+
 
 settings = Settings()