tune(friends): shorten presence auto-refresh 60s -> 15s

The 60s cadence predated the self-heal fix. 15s keeps the friends list
tracking online/in-room/offline changes more closely; each pass is still
just one short connection per friend, so the cost is small at typical
friend-list sizes. The manual Rescan button covers anything faster.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/core/mod.rs

@@ -966,10 +966,12 @@ async fn persist_and_emit_friends(
         .await;
 }
 
-/// How often the outbound presence scheduler refreshes friends' status. Slow on
-/// purpose — presence is best-effort, not real-time, and each pass opens a short
-/// connection per friend.
-const PING_INTERVAL: Duration = Duration::from_secs(60);
+/// How often the outbound presence scheduler refreshes friends' status. Each pass
+/// opens one short connection per friend with a saved address, so the cost scales
+/// with friend-count, not a fixed per-tick cost. 15s keeps the list feeling live
+/// without aggressively probing peers for a best-effort signal; the manual Rescan
+/// button covers the "update now" case below this interval.
+const PING_INTERVAL: Duration = Duration::from_secs(15);
 /// Delay before the FIRST presence pass, so the endpoint's background `online()`
 /// has a moment to finish (otherwise the first probes fail and friends flash offline).
 const PING_STARTUP_DELAY: Duration = Duration::from_secs(3);