feat(core): implement event outbox persistence

Add domain_events migration and SeaORM entity. Modify EventBus::publish
to persist events before broadcasting (best-effort: DB failure logs
warning but still broadcasts in-memory). Update all 19 publish call
sites across 4 crates to pass db reference.

Add outbox relay background task that polls pending events every 5s
and re-broadcasts them, ensuring no events are lost on server restart.

crates/erp-server/migration/src/lib.rs

@@ -30,6 +30,7 @@ mod m20260414_000027_fix_unique_indexes_soft_delete;
 mod m20260414_000028_add_standard_fields_to_tokens;
 mod m20260414_000029_add_standard_fields_to_process_variables;
 mod m20260415_000030_add_version_to_message_tables;
+mod m20260416_000031_create_domain_events;
 
 pub struct Migrator;
 
@@ -67,6 +68,7 @@ impl MigratorTrait for Migrator {
             Box::new(m20260414_000028_add_standard_fields_to_tokens::Migration),
             Box::new(m20260414_000029_add_standard_fields_to_process_variables::Migration),
             Box::new(m20260415_000030_add_version_to_message_tables::Migration),
+            Box::new(m20260416_000031_create_domain_events::Migration),
         ]
     }
 }