ops: Signal disk-tier attach pending vs ready

Large disk caches can look memory-only/broken during async DiskFS attach.
Expose disk_tier_ready in /metrics, pending/ready logs for disk-only and
mixed modes, and X-SteamCache-Disk-Tier on /lancache-heartbeat. GetMetrics
skips blocking disk.Size() while attach is pending so /metrics stays usable.

Closes #33

steamcache/handler.go

@@ -56,6 +56,15 @@ func (sc *SteamCache) handleSpecialEndpoints(w http.ResponseWriter, r *http.Requ
 		logger.Logger.Debug().
 			Str("client_ip", clientIP).
 			Msg("LanCache heartbeat request")
+		diskTier := "disabled"
+		if sc.disk != nil {
+			if sc.metrics.GetDiskTierReady() == 1 {
+				diskTier = "ready"
+			} else {
+				diskTier = "pending"
+			}
+		}
+		w.Header().Add("X-SteamCache-Disk-Tier", diskTier)
 		w.Header().Add("X-LanCache-Processed-By", "SteamCache2")
 		w.WriteHeader(http.StatusNoContent)
 		_, _ = w.Write(nil) // client write error ignored (heartbeat path; nil write is no-op)