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/ws/chat.py

@@ -6,8 +6,10 @@ from sqlalchemy import select
 from sqlalchemy.ext.asyncio import AsyncSession
 
 from app.database import get_db
-from app.models import RoomMembership, User
+from app.models import Room, RoomMembership, User
 from app.services.message_service import create_message
+from app.services.push_service import send_push_to_user
+from app.ws.connection_manager import ConnectionManager
 
 router = APIRouter(tags=["ws"])
 
@@ -29,6 +31,31 @@ async def _is_room_member(db: AsyncSession, room_id: uuid.UUID, user_id: uuid.UU
     return result.scalar_one_or_none() is not None
 
 
+async def _notify_offline_members(
+    db: AsyncSession,
+    manager: ConnectionManager,
+    room_id: uuid.UUID,
+    sender: User,
+    content: str,
+) -> None:
+    result = await db.execute(
+        select(RoomMembership.user_id).where(RoomMembership.room_id == room_id)
+    )
+    member_ids = {row[0] for row in result.all()}
+    offline_ids = member_ids - manager.connected_user_ids(room_id)
+    if not offline_ids:
+        return
+
+    room = await db.get(Room, room_id)
+    payload = {
+        "title": f"#{room.name}" if room else "New message",
+        "body": f"{sender.username}: {content}"[:120],
+        "room_id": str(room_id),
+    }
+    for user_id in offline_ids:
+        await send_push_to_user(db, user_id, payload)
+
+
 @router.websocket("/ws/chat")
 async def chat_endpoint(websocket: WebSocket, db: AsyncSession = Depends(get_db)) -> None:
     user_id_raw = websocket.session.get("user_id")
@@ -63,7 +90,7 @@ async def chat_endpoint(websocket: WebSocket, db: AsyncSession = Depends(get_db)
                         {"type": "error", "detail": "Not a member of this room"}
                     )
                     continue
-                manager.join(envelope.room_id, websocket)
+                manager.join(envelope.room_id, websocket, user.id)
                 joined_rooms.add(envelope.room_id)
                 await websocket.send_json({"type": "joined", "room_id": str(envelope.room_id)})
 
@@ -100,6 +127,9 @@ async def chat_endpoint(websocket: WebSocket, db: AsyncSession = Depends(get_db)
                         "created_at": message.created_at.isoformat(),
                     },
                 )
+                await _notify_offline_members(
+                    db, manager, envelope.room_id, user, envelope.content
+                )
 
             else:
                 await websocket.send_json(