fix(drone-airspace): show stale FAA snapshots as stale and cap them at a day

The source dropped the proxy's stale flag and fetch time, so the layer row
read nominal and freshly updated through an outage, and the proxy served its
last snapshot indefinitely. Carry stale and fetchedAt through the source as
Fire Perimeters does, date a stale snapshot by its fetch, and stop serving a
snapshot more than 24 hours old.

server/providers/droneAirspace.js

@@ -34,6 +34,8 @@ const API_URL =
 const MAX_PAGES = 5;
 const MIB = 1024 * 1024;
 const SNAPSHOT_TTL_MS = 3_600_000;
+/** Longest a failed refresh keeps serving the last snapshot, marked stale. */
+const STALE_MAX_MS = 24 * 60 * 60 * 1000;
 
 const exceededTransferLimit = (payload) =>
   payload?.exceededTransferLimit === true ||
@@ -100,7 +102,9 @@ export function droneAirspaceProxy({
       });
       return { value: await promise, stale: false };
     } catch (error) {
-      if (previous) return { value: previous.value, stale: true };
+      // A failed refresh serves the last snapshot, marked stale, for a day.
+      if (previous && now() - previous.savedAt <= STALE_MAX_MS)
+        return { value: previous.value, stale: true };
       throw error;
     }
   }