Make archiving a room actually affect existing members (#57)

is_archived was previously only exposed on the admin-only AdminRoom
schema and checked in one place (excluding a room from Browse rooms) --
for anyone already a member it was a complete no-op: still in their
sidebar, still fully postable, no indication anywhere it was archived.

Expose is_archived on the regular RoomRead/MyRoomItem schemas, drop
archived rooms from the sidebar list (while keeping them directly
reachable via URL so history stays readable), and reject new messages
in one -- both the WS "message" handler and incoming webhooks -- with a
clear "archived and read-only" response instead of silently no-op'ing
or a confusing membership error.

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

backend/app/ws/chat.py

@@ -6,7 +6,7 @@ from sqlalchemy import select
 from sqlalchemy.ext.asyncio import AsyncSession
 
 from app.database import get_db
-from app.models import ApiToken, Message, MessageFile, MessageImage, RoomMembership, User
+from app.models import ApiToken, Message, MessageFile, MessageImage, Room, RoomMembership, User
 from app.services.bot_service import resolve_token
 from app.services.message_events import (
     broadcast_dm_presence_update,
@@ -49,6 +49,11 @@ 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 _is_room_archived(db: AsyncSession, room_id: uuid.UUID) -> bool:
+    room = await db.get(Room, room_id)
+    return room is not None and room.is_archived
+
+
 def _missing_scope(api_token: ApiToken | None, scope: str) -> bool:
     return api_token is not None and scope not in api_token.scopes
 
@@ -186,6 +191,15 @@ async def chat_endpoint(websocket: WebSocket, db: AsyncSession = Depends(get_db)
                             {"type": "error", "detail": "Not a member of this room"}
                         )
                         continue
+                    if await _is_room_archived(db, envelope.room_id):
+                        # #57: history stays fully readable (joining/reading
+                        # an archived room's channel is untouched above),
+                        # this is the one gate that actually makes archiving
+                        # do something for people who were already members.
+                        await websocket.send_json(
+                            {"type": "error", "detail": "This room has been archived and is read-only"}
+                        )
+                        continue
                     image_id = None
                     if envelope.image_id is not None:
                         image = await db.get(MessageImage, envelope.image_id)