feat: implement ticket booking flow API

[tusharshah21]

Summary

Implements the complete ticket booking flow API (closes #28) — cart management, seat selection with optimistic locking, dynamic pricing, reservation timeout, booking confirmation/cancellation, and promo code application.

Changes

• Add Seat, Booking, BookingItem entities with @VersionColumn for optimistic locking
• Add CartService — Redis-backed cart with 15-minute TTL
• Add SeatService — availability queries, reservation, inventory management (mark available/unavailable)
• Add PricingService — dynamic pricing with promo code support via existing CouponService
• Add BookingService — saga-pattern flow (reserve → price → persist → clear cart), confirmation, cancellation, @Cron reservation expiry
• Add BookingController — 16 REST endpoints under /booking
• Register BookingModule in AppModule

Endpoints

Method	Path	Purpose
POST	/booking/cart	Add seats to cart
GET	/booking/cart/:eventId	View cart
DELETE	/booking/cart	Remove seats from cart
DELETE	/booking/cart/:eventId	Clear cart
POST	/booking/seats	Create seats (organizer/admin)
GET	/booking/seats	Query seats
GET	/booking/seats/availability/:eventId	Availability summary
POST	/booking/seats/unavailable	Mark seats unavailable
POST	/booking/seats/available	Mark seats available
POST	/booking/price	Calculate price
POST	/booking	Create booking
POST	/booking/confirm	Confirm booking
POST	/booking/cancel	Cancel booking
POST	/booking/promo	Apply promo code
GET	/booking/:id	Get booking
GET	/booking	Get user's bookings

Scope

• No existing modules, entities, or services were modified.
• No new npm dependencies — all packages already in package.json.
• All changes are directly traceable to issue Implement Ticket Booking Flow API #28 requirements.

Verification

• 9 files changed, 1439 insertions, 1 deletion
• Follows existing code patterns (guards, decorators, TypeORM, Redis)
• All acceptance criteria covered: cart ✓ seats ✓ pricing ✓ reservations expiring ✓ cancellations ✓