refactor: move transaction commit from repository to service layer

[Besthope-Official]

Related Issue

Closes #112

Summary of Changes

Removed transaction commit from repository layer and moved it to service layer:

• AuditRepository.create_audit_log(): Removed await self.session.commit() at src/audit/service.py:39
• AuditService.log(): Added await self.repository.session.commit() and await self.repository.session.rollback() for transaction control
• Updated tests/audit/test_service.py to remove commit assertion from repository test

Repository layer no longer controls transaction boundaries. Service layer manages commit/rollback, enabling future atomic operations across multiple repositories.

Breaking Changes

N/A

Checklist

• Issue discussion completed before opening PR
• Scope is small and focused (single feature/fix)
• All functions have full type annotations
• Async/await used for all I/O operations
• Tests added for new behaviors