[MR Train 2/6] Refactor: Implement database layer with JPA

MR Train Overview

This is MR 2 of 6 in a stacked MR train to refactor orders-service.

Train Structure

1. [MR 1/6] Foundation (!2 (merged))
2. [MR 2/6] Database layer ← YOU ARE HERE
3. [MR 3/6] Service layer (depends on MR 2)
4. [MR 4/6] API endpoints (depends on MR 3)
5. [MR 5/6] Integration tests (depends on MR 4)
6. [MR 6/6] Performance optimizations (depends on MR 5)

This MR: Database Layer

Implements the database persistence layer using Spring Data JPA.

Changes

• Created OrderRepositoryImpl with Spring Data JPA
• Added JPA entity mappings (OrderEntity, OrderItemEntity)
• Created Flyway migration scripts for orders and order_items tables
• Added proper indexes for query optimization

Dependencies

⚠️ Depends on: !2 (merged) (MR 1/6 - Foundation)

This MR is based on the refactor/step1-foundation branch and must be merged AFTER MR 1.

Review Notes

• All queries use proper indexing
• Entity-to-domain conversion is handled cleanly
• Migration scripts are idempotent

@sabrina_farmer Can you review the JPA implementation?