feat: maintenance/blocked date ranges with booking overlap warning

- DB migration 020: add maintenance_from, maintenance_to to rooms
- Room type + RoomPayload: maintenanceFrom, maintenanceTo fields
- Context menu: clicking "На ремонт"/"Закрыт" expands inline date picker
  (С / По + "Без срока" checkbox), confirms with "Применить" button
- Calendar: stripe overlay covers only the maintenance date range
  (full row if no dates set, partial if date range specified)
- Booking creation: if drag-selected dates overlap with maintenance period,
  shows warning dialog with "Отмена" / "Всё равно забронировать"

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

backend/migrations/020_room_maintenance_dates.sql

@@ -0,0 +1,3 @@
+-- Add maintenance/blocked date range columns to rooms
+ALTER TABLE rooms ADD COLUMN IF NOT EXISTS maintenance_from DATE;
+ALTER TABLE rooms ADD COLUMN IF NOT EXISTS maintenance_to   DATE;