Add per-user ad_tag with global fallback and hot-reload

- Per-user ad_tag in [access.user_ad_tags], global fallback in general.ad_tag
- User tag overrides global; if no user tag, general.ad_tag is used
- Both general.ad_tag and user_ad_tags support hot-reload (no restart)

src/main.rs

@@ -448,7 +448,7 @@ async fn main() -> std::result::Result<(), Box<dyn std::error::Error>> {
             info!("Middle-proxy STUN probing disabled by network.stun_use=false");
         }
 
-        // ad_tag (proxy_tag) for advertising
+        // Global ad_tag (pool default). Used when user has no per-user tag in access.user_ad_tags.
         let proxy_tag = config
             .general
             .ad_tag