feat(api): add GET /v1/stats/users/active-ips endpoint

Lightweight endpoint that returns only users with active TCP connections
and their IP addresses. Calls only get_active_ips_for_users() without
collecting recent IPs or building full UserInfo, significantly reducing
CPU and memory overhead compared to /v1/stats/users.

src/api/model.rs

@@ -442,6 +442,12 @@ pub(super) struct UserInfo {
     pub(super) links: UserLinks,
 }
 
+#[derive(Serialize)]
+pub(super) struct UserActiveIps {
+    pub(super) username: String,
+    pub(super) active_ips: Vec<IpAddr>,
+}
+
 #[derive(Serialize)]
 pub(super) struct CreateUserResponse {
     pub(super) user: UserInfo,