feat(core): add audit logging to all mutation operations

Create audit_log SeaORM entity and audit_service::record() helper.
Integrate audit recording into 35 mutation endpoints across all modules:
- erp-auth: user/role/organization/department/position CRUD (15 actions)
- erp-config: dictionary/menu/setting/numbering_rule CRUD (15 actions)
- erp-workflow: definition/instance/task operations (8 actions)
- erp-message: send/system/mark_read/delete (5 actions)

Uses fire-and-forget pattern — audit failures logged but non-blocking.

crates/erp-core/src/lib.rs

@@ -1,4 +1,6 @@
 pub mod audit;
+pub mod audit_service;
+pub mod entity;
 pub mod error;
 pub mod events;
 pub mod module;