Add the ability to delete a message (#53)

Message.deleted_at has existed since the initial schema but was never
wired up -- no WS envelope, no permission check, no frontend concept of
it at all. Soft delete, author-only (mirrors the existing edit
permission exactly): content and any attached image/file are cleared
and the underlying MessageImage/MessageFile row and stored file are
actually removed, not just detached, so the message becomes a "This
message was deleted" tombstone with nothing left to recover through a
stale attachment URL. A deleted message can no longer be edited or
reacted to.

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

backend/app/routers/rooms.py

@@ -433,6 +433,7 @@ async def get_room_messages_endpoint(
             reactions=reactions_by_message.get(m.id, []),
             created_at=m.created_at,
             edited_at=m.edited_at,
+            deleted_at=m.deleted_at,
         )
         for m in messages
     ]