feat: tariff selector in BookingModal

- Migration 050: add tariff_id FK to bookings table
- Backend: accept tariff_id in POST/PATCH bookings
- Frontend: load active tariffs in BookingModal, show tariff dropdown
- Price calculation applies tariff modifier (percent or fixed) before discount
- tariffId forwarded through CalendarPage → api.bookings.create/update

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

backend/migrations/050_booking_tariff_id.sql

@@ -0,0 +1,3 @@
+-- Add tariff reference to bookings
+ALTER TABLE bookings
+  ADD COLUMN IF NOT EXISTS tariff_id UUID REFERENCES tariffs(id) ON DELETE SET NULL;