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>

frontend/src/components/ChatPane.tsx

@@ -272,6 +272,7 @@ export function ChatPane({
         members={members}
         rooms={rooms}
         disabled={!connected}
+        archived={room.is_archived}
         onSend={send}
       />
     </section>