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-server/migration/src/lib.rs

@@ -25,6 +25,7 @@ mod m20260412_000022_create_process_variables;
 mod m20260413_000023_create_message_templates;
 mod m20260413_000024_create_messages;
 mod m20260413_000025_create_message_subscriptions;
+mod m20260413_000026_create_audit_logs;
 
 pub struct Migrator;
 
@@ -57,6 +58,7 @@ impl MigratorTrait for Migrator {
             Box::new(m20260413_000023_create_message_templates::Migration),
             Box::new(m20260413_000024_create_messages::Migration),
             Box::new(m20260413_000025_create_message_subscriptions::Migration),
+            Box::new(m20260413_000026_create_audit_logs::Migration),
         ]
     }
 }