refactor(push): clean up legacy push_config and w_push_events hardcoded insertions

- Remove ConfigKeyPushConfig and delete legacy push_config query logic from EventTrigger.Trigger.
- Simplify the push event notification dispatching engine to rely purely on database custom channels.
- Remove w_push_events hardcoded INSERT statement from migration files, letting SyncEvents handle default event registration.
- Rewrite unit tests to use model.PushChannel instead of push_config.

internal/model/system_configs.go

@@ -49,7 +49,6 @@ const (
 	ConfigKeyLoginSessionTTLHours             = "login_session_ttl_hours"             // 登录会话过期时间 (小时，0表示浏览器关闭后自动退出登录，-1表示永不过期)
 	ConfigKeyUpdateUpstreamRepository         = "update_upstream_repository"          // GitHub Actions Release 上游仓库
 	ConfigKeyStorageConfig                    = "storage_config"                      // 文件存储配置 (JSON)
-	ConfigKeyPushConfig                       = "push_config"                         // 通知推送渠道配置 (JSON)
 )
 
 const (