fix(perimeters): report a stale WFIGS snapshot as stale, and stop serving it after a day

When WFIGS was unreachable the proxy served its last snapshot, marked stale
(X-Data-Stale, body.stale), with no age limit, and the client source kept
only the rows. The layer then read healthy with a fresh update time and
get_fire_perimeters reported the perimeters as current; three days into an
outage it still said "1 mapped wildfire perimeter in the requested box."
The source now offers getSnapshotDetail ({rows, stale, fetchedAt}); the
layer reports stale with the snapshot's fetch time, the tool adds stale,
retrieved_at and "(data may be stale)", and the proxy serves a stale
snapshot for at most 24 hours, as the GNSS proxy caps its own.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

server/providers/firePerimeters.js

@@ -39,6 +39,8 @@ const API_URL =
 // The service caps a single response at its maxRecordCount (2000); a peak
 // season can exceed that, so follow exceededTransferLimit with offset pages.
 const MAX_PAGES = 5;
+/** Longest a failed refresh keeps serving the last perimeter snapshot, marked stale. */
+const STALE_MAX_MS = 24 * 60 * 60 * 1000;
 
 const exceededTransferLimit = (payload) =>
   payload?.exceededTransferLimit === true ||
@@ -165,7 +167,14 @@ export function firePerimetersProxy({
       });
       return { value: await promise, stale: false };
     } catch (error) {
-      if (previous) return { value: previous.value, stale: true };
+      // A failed refresh serves the last answer, marked stale. A perimeter
+      // snapshot only for a day: older than that, it is no picture of
+      // current fires. (InciWeb entries keep their previous behaviour.)
+      if (
+        previous &&
+        (key !== 'perimeters' || now() - previous.savedAt <= STALE_MAX_MS)
+      )
+        return { value: previous.value, stale: true };
       throw error;
     }
   }