swarm: re-announce promptly when the swarm is starved

The peer top-up loop re-announced to trackers only at the full advertised
interval (~30 min), so a torrent that dropped to a handful of peers would
sit there for up to half an hour with only the 5-minute DHT refresh to
help — looking like the re-announce system was dead.

Now, when fewer than LOW_PEER_THRESHOLD (10) peers are connected, the next
tracker announce is scheduled at the tracker's min_interval floor (never
below 60s) instead of the full interval, so a thin swarm actually tries to
recover. Once peers recover the full interval is used again. Capture the
tracker's min_interval (was being dropped) to stay announce-compliant.

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

src/discovery/tracker_client.c

@@ -51,6 +51,7 @@ static naut_err collect_peers(const tracker_peer *peers, size_t count,
                               const tracker_announce_response *resp,
                               naut_tracker_response *out) {
     out->interval = (int32_t)resp->interval;
+    out->min_interval = (int32_t)resp->min_interval;
     out->seeders = (int32_t)resp->complete;
     out->leechers = (int32_t)resp->incomplete;
     out->peers = NULL;