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/lib/api.ts

@@ -691,6 +691,7 @@ export interface BookingPayload {
   roomId?: string; guestName?: string; guestEmail?: string; guestPhone?: string
   checkIn?: string; checkOut?: string; adults?: number; children?: number
   status?: string; source?: string; totalAmount?: number; paidAmount?: number; notes?: string
+  tariffId?: string | null
 }
 
 function toBookingPayload(b: Partial<BookingPayload>): Record<string, unknown> {
@@ -708,6 +709,7 @@ function toBookingPayload(b: Partial<BookingPayload>): Record<string, unknown> {
   if (b.totalAmount !== undefined) out.total_amount = b.totalAmount
   if (b.paidAmount  !== undefined) out.paid_amount  = b.paidAmount
   if (b.notes       !== undefined) out.notes        = b.notes
+  if (b.tariffId    !== undefined) out.tariff_id    = b.tariffId
   return out
 }