feat(core): EventBus dead-letter + consume_with_retry 辅助函数

- 新增 dead_letter_events 表 + Entity
- consume_with_retry: 幂等检查 + 成功标记 + 失败转入 dead-letter
- insert_dead_letter: 写入失败事件供后续排查和手动重试

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

@@ -90,6 +90,7 @@ mod m20260427_000087_audit_logs_hash_chain;
 mod m20260428_000088_rls_policy_strict;
 mod m20260428_000089_blind_indexes;
 mod m20260428_000090_critical_alerts;
+mod m20260428_000091_dead_letter_events;
 
 pub struct Migrator;
 
@@ -187,6 +188,7 @@ impl MigratorTrait for Migrator {
             Box::new(m20260428_000088_rls_policy_strict::Migration),
             Box::new(m20260428_000089_blind_indexes::Migration),
             Box::new(m20260428_000090_critical_alerts::Migration),
+            Box::new(m20260428_000091_dead_letter_events::Migration),
         ]
     }
 }