feat: YooKassa webhook, payment timeout, reserved status, BookingConfirmPage

- Add YooKassa webhook handler (POST /api/yookassa/webhook) that updates
  booking to confirmed+paid on payment.succeeded, cancelled on payment.canceled
- Add public status endpoint GET /api/online-bookings/:id for return URL polling
- Add 'reserved' booking status (violet) shown in calendar while awaiting payment
- Add 'website' to BookingSource type
- Payment timeout job (every 2 min) auto-cancels expired unpaid bookings
- Widget setting: "Время ожидания оплаты" (5–60 min, default 15)
- BookingConfirmPage at /booking-confirm/:id — polls status, countdown timer,
  shows success/pending/cancelled state
- Widget bookings now use status='reserved' instead of 'inquiry' when YooKassa
  is configured; set to 'confirmed' by webhook on payment.succeeded
- Migration 070: add 'reserved' to bookings status constraint + payment_expires_at

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

backend/migrations/070_reserved_status_payment_expires.sql

@@ -0,0 +1,9 @@
+-- Migration 070 — Add 'reserved' booking status + payment_expires_at for widget bookings
+
+-- Add 'reserved' to bookings status constraint
+ALTER TABLE bookings DROP CONSTRAINT IF EXISTS bookings_status_check;
+ALTER TABLE bookings ADD CONSTRAINT bookings_status_check
+  CHECK (status IN ('inquiry','confirmed','checked_in','checked_out','cancelled','no_show','reserved'));
+
+-- Add payment expiry column to online_bookings
+ALTER TABLE online_bookings ADD COLUMN IF NOT EXISTS payment_expires_at TIMESTAMPTZ;