fix(cctv): warn once per failed background refresh; test the single flight

Attach the background-refresh warning where the refresh starts, so a
failed refresh logs once instead of once per request served meanwhile.
The test now checks that requests during a refresh share it, and turns
the hang off before releasing the fetches so a retrying loader can't
stall it.

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

src/data/cctvCatalogCap.test.mjs

@@ -217,7 +217,18 @@ test('an expired catalog is served at once while a slow pack refreshes in the ba
       ['old-0', 'old-1', 'old-2'],
     );
 
+    // Requests during the refresh share it rather than starting another.
+    const pause = () => new Promise((resolve) => setTimeout(resolve, 50));
+    await pause();
+    const inFlight = hung.length;
+    assert.ok(inFlight > 0, 'the refresh reached the hanging upstreams');
+    await getSources();
+    await getSources();
+    await pause();
+    assert.equal(hung.length, inFlight, 'only one refresh ran');
+
     // When the slow pack gives up, the refreshed catalog replaces the old one.
+    hang = false;
     for (const giveUp of hung) giveUp();
     for (let i = 0; i < 50; i += 1) {
       const ids = (await getSources()).map((s) => s.id);