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.

src/layers/droneAirspace/source.js

@@ -4,20 +4,35 @@ import { readResponseJsonCapped } from '../../sources/httpBody.js';
 export function createDroneAirspaceSource({
   fetchImpl = (...args) => globalThis.fetch(...args),
 } = {}) {
+  /**
+   * The rows with how fresh they are: the proxy answers from its last
+   * snapshot, marked stale, while the FAA service is unreachable.
+   * @returns {Promise<{rows: object[], stale: boolean, fetchedAt: number|null}>}
+   */
+  async function getSnapshotDetail({ signal } = {}) {
+    signal?.throwIfAborted();
+    const response = await fetchImpl('/api/drone-airspace', { signal });
+    if (!response.ok) throw new Error(`FAA SUA HTTP ${response.status}`);
+    const payload = await readResponseJsonCapped(
+      response,
+      80 * 1024 * 1024,
+      signal,
+    );
+    signal?.throwIfAborted();
+    if (!Array.isArray(payload?.rows))
+      throw new Error('Malformed drone airspace snapshot');
+    return {
+      rows: payload.rows,
+      stale: payload.stale === true,
+      fetchedAt: Number.isFinite(payload.fetchedAt) ? payload.fetchedAt : null,
+    };
+  }
+
   return {
-    async getSnapshot({ signal } = {}) {
-      signal?.throwIfAborted();
-      const response = await fetchImpl('/api/drone-airspace', { signal });
-      if (!response.ok) throw new Error(`FAA SUA HTTP ${response.status}`);
-      const payload = await readResponseJsonCapped(
-        response,
-        80 * 1024 * 1024,
-        signal,
-      );
-      signal?.throwIfAborted();
-      if (!Array.isArray(payload?.rows))
-        throw new Error('Malformed drone airspace snapshot');
-      return payload.rows;
+    /** The zone rows (see getSnapshotDetail for staleness). */
+    async getSnapshot(options = {}) {
+      return (await getSnapshotDetail(options)).rows;
     },
+    getSnapshotDetail,
   };
 }