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/data/droneAirspaceProxy.test.mjs

@@ -213,3 +213,25 @@ test('a cold upstream failure is 502, not an empty snapshot', async () => {
   assert.equal(res.status, 502);
   assert.equal(res.body.error, 'drone_airspace_unavailable');
 });
+
+test('a stale snapshot is served for at most a day, then refused', async () => {
+  let now = 1_000_000_000;
+  let up = true;
+  const request = install({
+    now: () => now,
+    fetchImpl: async () => {
+      if (!up) throw new Error('FAA unreachable');
+      return Response.json({ features: [feature(7)] });
+    },
+  });
+  const fresh = await request();
+  assert.equal(fresh.status, 200);
+  up = false;
+  now += 23 * 60 * 60 * 1000;
+  const stale = await request();
+  assert.equal(stale.status, 200);
+  assert.equal(stale.body.stale, true);
+  assert.equal(stale.body.fetchedAt, fresh.body.fetchedAt);
+  now += 2 * 60 * 60 * 1000;
+  assert.equal((await request()).status, 502, 'a day-old snapshot is refused');
+});