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/schemas/room.py

@@ -27,6 +27,11 @@ class RoomRead(BaseModel):
     description: str | None
     is_private: bool
     is_dm: bool
+    # #57: previously only exposed on the admin-only AdminRoom schema, so a
+    # member of an archived room had no way to even know it was archived --
+    # the flag was set server-side but had no effect on their own view of
+    # the room at all.
+    is_archived: bool
     owner_id: uuid.UUID
     created_at: datetime