fix: seedIdentities 硬编码配置名导致admin身份丢失

- 迁移后配置名 qq→obv11-main，但seed用["obv11"]查不到
- 改为遍历store.List()所有配置，不再硬编码名字
- admin消息不再被当普通人处理而丢失回复

Co-Authored-By: Claude <noreply@anthropic.com>

backend/platform-bridge/cmd/main.go

@@ -992,13 +992,12 @@ func seedIdentities(m *bridge.IdentityMapper, store *config.Store, adminNickname
 		}
 	}
 
-	// From stored platform configs (admin_uids field).
-	for _, name := range []string{"obv11", "telegram", "webhook", "wechat", "feishu", "discord"} {
-		stored, _ := store.Get(name)
-		if stored == nil {
+	// From stored platform configs (admin_uids field) — iterate all configs.
+	for _, cfg := range store.List() {
+		if !cfg.Enabled {
 			continue
 		}
-		syncAdminUIDs(m, name, stored.Fields, adminNickname)
+		syncAdminUIDs(m, cfg.Platform, cfg.Fields, adminNickname)
 	}
 }