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>

src/pages/CalendarPage.tsx

@@ -166,6 +166,7 @@ export function CalendarPage() {
         adults: data.adults, children: data.children,
         status: data.status, source: data.source,
         totalAmount: data.totalAmount, paidAmount: data.paidAmount, notes: data.notes,
+        tariffId: data.tariffId,
       })
       setBookings(prev => [...prev, created])
       send({ type: 'booking:created', booking: created })
@@ -186,6 +187,7 @@ export function CalendarPage() {
         adults: data.adults, children: data.children,
         status: data.status, source: data.source,
         totalAmount: data.totalAmount, notes: data.notes,
+        tariffId: data.tariffId,
       })
       setBookings(prev => prev.map(b => b.id === id ? updated : b))
       send({ type: 'booking:updated', booking: updated })