[Feature] Implement Atomic Reservation Operations #12

Problem

Concurrent inventory reservations may cause overselling (negative stock).

Impact

• Inventory inconsistency
• Reservations fail unexpectedly
• Stock inaccuracy

Solution

Use MongoDB transactions for reserve operations:

• Atomic inventory deduction
• Reservation document creation
• Rollback on failure

Acceptance Criteria

• MongoDB transactions implemented
• Overselling prevented
• Concurrency tests added
• Reservation failure handling