fix: deduplicate background refresh with per-domain guard

Multiple stale queries for the same domain now spawn only one background
refresh. A HashSet<(String, QueryType)> on ServerCtx tracks in-flight
refreshes; subsequent stale hits for the same key skip the spawn.

src/main.rs

@@ -285,6 +285,7 @@ async fn main() -> numa::Result<()> {
             config.cache.min_ttl,
             config.cache.max_ttl,
         )),
+        refreshing: Mutex::new(std::collections::HashSet::new()),
         stats: Mutex::new(ServerStats::new()),
         overrides: RwLock::new(OverrideStore::new()),
         blocklist: RwLock::new(blocklist),