Refactor: Split services.rs into dedicated service modules

[Samuel1505]

[BA-01] Refactor: Split services.rs into dedicated service modules

Summary

closes #407

• Extracted ProductService, EventService, UserService, ApiKeyService, SyncService, and RecallService from the 1282-line monolithic services.rs into individual files under backend/src/services/
• Fixed a correctness issue: cache helper methods (invalidate_product_cache, invalidate_global_stats) were incorrectly placed inside trait impl blocks; they are now in inherent impl blocks where they belong
• Reduced services.rs to 40 lines of pub mod declarations and pub use re-exports — all existing import paths remain unchanged
• Added missing re-exports for BatchService, BatchRepository, SupplierService, IoTService, QualityService, and RegulatoryService, which were referenced in handlers but not previously re-exported

New files

File	Lines	Contents
services/product.rs	266	ProductService + ProductRepository impl
services/event.rs	169	EventService + EventRepository impl
services/user.rs	178	UserService + UserRepository impl
services/api_key.rs	143	ApiKeyService + ApiKeyRepository impl
services/sync.rs	64	SyncService
services/recall.rs	343	RecallService

Test plan

• cargo check passes with no new errors
• All handler paths that call into services resolve correctly (no broken imports)
• crate::services::ApiKeyService::hash_api_key / generate_api_key still callable as static methods
• crate::services::UserService::hash_password still callable as a static method
• crate::services::BatchRepository trait is accessible from handlers/batch.rs

Relates to

Closes #407

[vercel]

@Samuel1505 is attempting to deploy a commit to the Ebeleokolo's projects Team on Vercel.

A member of the Team first needs to authorize it.

[Lynndabel]

welldone @Samuel1505