Scraper: stop walking the whole library once a minute for status polls

The reader polled /api/state every 15 s; each call rebuilt the scraper's library
summary when it was over 60 s old, reading every chapter.json (on the NAS about
5,300 opens / 16,000 NFS calls and 1.3 s CPU per walk, once a minute, plus 115 KB
per poll). The reader now polls GET /api/revision (falls back to /api/state on an
older scraper), and the summary is rebuilt after a library change or after 15
minutes. Release-date writes invalidate it too.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

scraper/test/inventory-cache.test.js

@@ -80,6 +80,23 @@ test('background inventory failures are visible without breaking controls or lau
   assert.equal(scans, 2);
 });
 
+test('status polling walks the library again only after a change or after 15 minutes', async () => {
+  let scans = 0;
+  const service = fixture(async () => { scans++; return inventory('Demo'); });
+  await service.libraryInventory(true);
+  service.inventoryCachedAt = Date.now() - 10 * 60000; service.inventoryAttemptedAt = 0;
+  for (let n = 0; n < 5; n++) await service.publicState();
+  assert.equal(scans, 1);
+  assert.deepEqual(service.revision(), { publishedRevision: 0, status: 'paused', paused: true });
+  assert.equal(scans, 1);
+  service.invalidateInventory();
+  await service.publicState(); await service.inventoryRefresh;
+  assert.equal(scans, 2);
+  service.inventoryCachedAt = Date.now() - 16 * 60000; service.inventoryAttemptedAt = 0;
+  await service.publicState(); await service.inventoryRefresh;
+  assert.equal(scans, 3);
+});
+
 test('dashboard polling reuses a recent snapshot while decisions still force a fresh scan', async () => {
   let scans = 0;
   const service = fixture(async () => { scans++; return inventory('Demo'); });