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/routers/rooms.py

@@ -147,6 +147,7 @@ async def list_rooms_endpoint(
             description=room.description,
             is_private=room.is_private,
             is_dm=room.is_dm,
+            is_archived=room.is_archived,
             owner_id=room.owner_id,
             created_at=room.created_at,
             is_member=is_member,
@@ -171,6 +172,7 @@ async def list_my_rooms_endpoint(
             description=room.description,
             is_private=room.is_private,
             is_dm=room.is_dm,
+            is_archived=room.is_archived,
             owner_id=room.owner_id,
             created_at=room.created_at,
             role=role,