[DB] Redesign booking schema and seat reservation logic

[phamvanson04]

Problem

Current booking flow can allow race conditions when multiple users reserve the same seat at nearly the same time.

Scope

• Redesign booking persistence schema for seat reservation safety
• Add seat-level locking strategy in booking transaction
• Improve booking grouping metadata for multi-seat reservations
• Update repository methods to support transactional locking
• Provide migration SQL guidance

Acceptance Criteria

• Prevent double-booking in concurrent requests
• Keep existing booking API behavior stable
• Provide backward-compatible DB migration script and indexes
• Booking flow remains transactional and consistent