Add PurchaseItem entity with status tracking (Pending/Ordered/Received),
supplier and job relationships. Add LockedAt timestamp to Job entity for
controlling editability after materials are ordered. Includes
PurchaseItemService (CRUD + bulk create), JobService Lock/Unlock methods,
EF Core migrations, and DI registration.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>