Payments: enable Razorpay checkout for 1:1 consultations behind FeatureFlags.payments

[teetangh]

Context

The schema-sync MVP ships read-first: booking requests are created without payment (bookingSource: REQUEST_SUBMITTED), consultants approve to APPROVED_PENDING_PAYMENT, and payment completes on the web. All checkout/payment surfaces are gated behind FeatureFlags.payments (false), with backend featureGate middleware returning 403 feature_disabled on checkout/, payments/, invoices/, webhooks/.

App Store compliance

• 1:1 person-to-person services (consultations, 1:1 subscriptions) may use purchase methods other than IAP per App Review Guideline 3.1.3(d) → Razorpay in-app is allowed for these.
• One-to-few/one-to-many (webinars, classes) must use IAP — tracked separately.
• Keep approved-pending-payment copy neutral for group programs (already handled in lib/domain/entities/booking/booking.dart).

Scope (priority order: Razorpay → Stripe → Dodo)

1. Re-enable direct checkout path in lib/features/booking/screens/booking_screen.dart (FeatureFlags.payments branch already preserves it)
2. Update checkout_repository.dart + webhook_handlers.dart for the synced schema: Payment BigInt paise amounts, PaymentLeg rows (CARD leg at minimum), clientIdempotencyKey, Refund/Dispute.amountPaise
3. Decide webhook ownership — see refactor(payments): Swiggy-style architecture - Web-only webhook processing with mobile read-only access #15 (web-only webhook processing) and Critical: Race condition risks when mobile and web backends run simultaneously #19 (race conditions when both backends run)
4. Pay-now button re-enable (booking_action_buttons.dart, my_bookings_screen.dart — already flag-gated)
5. Flip FeatureFlags.payments in lib/core/config/feature_flags.dart + backend/lib/config/feature_flags.dart together

Related: #15, #19, #20, #44, #45, #57