feat: group chat owner protection — prevent removing creator, transfer ownership before leaving

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/migrations/079_chat_room_owner.sql

@@ -0,0 +1,2 @@
+-- Group room owner (creator)
+ALTER TABLE chat_rooms ADD COLUMN IF NOT EXISTS created_by UUID REFERENCES users(id) ON DELETE SET NULL;