feat(core): add audit log infrastructure (Phase 6)

- Add audit_logs database migration with indexes
- Add AuditLog struct in erp-core with builder pattern
- Support old/new value tracking, IP address, user agent

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

crates/erp-core/src/lib.rs

@@ -1,3 +1,4 @@
+pub mod audit;
 pub mod error;
 pub mod events;
 pub mod module;