fix: chat — VARCHAR(10) overflow for notifications type, skeleton loading

- Migration 072: widen type column to VARCHAR(20), add partial unique index for general rooms
- Backend: replace brittle ON CONFLICT with explicit SELECT-then-INSERT ensureRoom helper
- Frontend: show room skeletons (Общий чат + Уведомления) during load instead of spinner,
  hide '0 чатов' subtitle when list is empty

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

backend/migrations/072_chat_room_type_length.sql

@@ -0,0 +1,6 @@
+-- Increase type column length to fit 'notifications'
+ALTER TABLE chat_rooms ALTER COLUMN type TYPE VARCHAR(20);
+
+-- Partial unique index for general room per hotel (needed for ON CONFLICT)
+CREATE UNIQUE INDEX IF NOT EXISTS uniq_chat_rooms_general_hotel
+  ON chat_rooms(hotel_id) WHERE type = 'general';