AI 托管账号支持筛选未托管

筛选只有「仅已托管」一个选项，想找还没开启的账号只能翻页。补上
managed=0：未托管既包括从未绑定的账号，也包括绑定后被关闭的账号，
运营要开启的正是这两类，所以放在一起。

测试覆盖三种状态，并断言两个筛选相加等于全部、没有重复计数。

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

im/backend/internal/app/admin_ai_agents.go

@@ -33,8 +33,14 @@ func (a *App) adminAIAgents(w http.ResponseWriter, r *http.Request) {
 		fail(w, http.StatusBadRequest, 20001, err.Error())
 		return
 	}
-	if strings.TrimSpace(r.URL.Query().Get("managed")) == "1" {
+	// "0" is not the absence of a filter here: an account is unmanaged both when
+	// it has no binding at all and when its binding is switched off, and the
+	// operator looking for accounts to turn on wants to see both.
+	switch strings.TrimSpace(r.URL.Query().Get("managed")) {
+	case "1":
 		where += " AND g.user_id IS NOT NULL AND g.enabled=1"
+	case "0":
+		where += " AND (g.user_id IS NULL OR g.enabled=0)"
 	}
 	// Same keyword shape as the user list: nickname or 星遇号.
 	if keyword := strings.TrimSpace(r.URL.Query().Get("keyword")); keyword != "" {