feat: chat — group chat creation (create-group view, backend endpoint, group room display)

backend/migrations/076_chat_group_rooms.sql

@@ -0,0 +1,4 @@
+-- Allow 'group' room type (drop old check constraint, add new one)
+ALTER TABLE chat_rooms DROP CONSTRAINT IF EXISTS chat_rooms_type_check;
+ALTER TABLE chat_rooms ADD CONSTRAINT chat_rooms_type_check
+  CHECK (type IN ('general', 'direct', 'notifications', 'group'));