Push a live signal when a user is added to a room (#26)

Previously GET /api/rooms/mine was only ever fetched once at app mount,
so a room added mid-session stayed invisible until a full page reload
-- add_member had no way to reach an already-open client at all.

Backend: ConnectionManager and Broadcaster (renamed from RoomBroadcaster)
now support per-user channels alongside the existing per-room ones, so a
signal can reach a user's socket even for a room they haven't joined
(and by definition can't have, until this fires). add_member publishes
a room_added event on the target user's channel.

Frontend: the WebSocket connection is no longer scoped to whichever
room is open -- ChatShellPage now owns one persistent connection for
the whole session (including while no room is open, which is exactly
when this bug showed), and ChatPane joins/leaves rooms on top of it.
A room_added event triggers a room-list refetch with no reload needed.

Verified end-to-end in the browser: a user sitting on the empty room
list saw a newly-added room appear live, then chatted in it normally.

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

backend/app/services/message_events.py

@@ -7,7 +7,7 @@ from app.models import Message, MessageFile, Room, RoomMembership, User
 from app.schemas.message import ReactionSummary
 from app.services.push_service import send_push_to_user
 from app.services.webhook_service import dispatch_event
-from app.ws.broadcaster import RoomBroadcaster
+from app.ws.broadcaster import Broadcaster
 from app.ws.presence import Presence
 
 
@@ -70,7 +70,7 @@ async def _message_payload(db: AsyncSession, message: Message, username: str) ->
 
 async def broadcast_new_message(
     db: AsyncSession,
-    broadcaster: RoomBroadcaster,
+    broadcaster: Broadcaster,
     presence: Presence,
     room_id: uuid.UUID,
     message: Message,
@@ -86,7 +86,7 @@ async def broadcast_new_message(
 
 
 async def broadcast_message_update(
-    db: AsyncSession, broadcaster: RoomBroadcaster, room_id: uuid.UUID, message: Message
+    db: AsyncSession, broadcaster: Broadcaster, room_id: uuid.UUID, message: Message
 ) -> None:
     payload = {
         "type": "message_update",
@@ -100,7 +100,7 @@ async def broadcast_message_update(
 
 
 async def broadcast_reaction_update(
-    broadcaster: RoomBroadcaster,
+    broadcaster: Broadcaster,
     room_id: uuid.UUID,
     message_id: uuid.UUID,
     reactions: list[ReactionSummary],
@@ -115,3 +115,15 @@ async def broadcast_reaction_update(
     # Deliberately no dispatch_event() call -- reactions don't get an
     # outgoing-webhook event type, matching the same scope cut made for
     # image uploads (see backend/README.md).
+
+
+async def broadcast_room_added(broadcaster: Broadcaster, user_id: uuid.UUID, room: Room) -> None:
+    """The only signal a user's open client gets that they were just added
+    to a room -- without it, GET /rooms/mine is only ever fetched once at
+    app mount, so a room added mid-session stays invisible until a full
+    reload. Published on the user's own channel rather than the room's,
+    since the whole point is reaching someone who hasn't joined that room's
+    channel yet (and by definition can't have)."""
+    await broadcaster.publish_to_user(
+        user_id, {"type": "room_added", "room_id": str(room.id)}
+    )