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/Sidebar.tsx

@@ -37,7 +37,11 @@ export function Sidebar({
     }
     return room.name.toLowerCase().includes(query)
   }
-  const filtered = rooms.filter(matchesQuery)
+  // #57: archived rooms keep flowing through in `rooms` (so a member who
+  // still has one open via a direct link resolves fine -- see ChatPane),
+  // but they're a dead end going forward, so they don't belong in the list
+  // you'd browse/search from.
+  const filtered = rooms.filter((r) => !r.is_archived).filter(matchesQuery)
   const directMessages = filtered.filter((r) => r.is_dm)
   const regularRooms = filtered.filter((r) => !r.is_dm)