Heartbeat honesty: SYNC badge reads refreshed_at, not grade-lock updated_at

The "SIGNAL LIVE vs STALE 8h" contradiction was a field mismatch, not a dead
pipeline. updated_at is the grade-LOCK time (advances only on a full snapshot,
5×/day — grades never change in-game, so it is intentionally stable). The
SYNC badge measured the 20-min intraday cadence (expected_interval_s=1200)
against that 5×/day field → structurally guaranteed STALE between slots even
when intraday refreshes lines perfectly.

- snapshotService: full snapshot now seeds refreshed_at at lock time
- intradayRefresh already bumps refreshed_at every ~20 min (unchanged)
- /api/snapshot/summary + GET /:sport now expose refreshed_at (was written to
  Redis but never serialized → no public liveness signal existed)
- LiveLayer SYNC badge measures freshness from refreshed_at (fallback updated_at)

Exposing refreshed_at also gives a public heartbeat probe: it advances every
intraday slot, so pipeline liveness is verifiable without container logs.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/integration/snapshotSummary.test.js

@@ -43,6 +43,28 @@ describe('GET /api/snapshot/summary', () => {
     expect(res.body.sports.nba).toBe(0); // off-season → honest zero
   });
 
+  test('refreshed_at is the freshness heartbeat, distinct from the grade-lock updated_at', async () => {
+    // updated_at = grade LOCK (5×/day, stable). refreshed_at = last intraday
+    // line refresh (~20 min). The SYNC badge measures against refreshed_at —
+    // measuring the 20-min cadence against the lock field caused "STALE 8h".
+    store['snapshot:mlb:latest'] = {
+      updated_at: '2026-07-10T18:00:00.000Z',       // graded at 2pm ET
+      refreshed_at: '2026-07-10T21:40:00.000Z',     // lines refreshed at 5:40pm ET
+      grades: [{ player: 'A' }],
+    };
+    const res = await request(mountApp()).get('/api/snapshot/summary');
+    expect(res.body.updated_at).toBe('2026-07-10T18:00:00.000Z');
+    expect(res.body.refreshed_at).toBe('2026-07-10T21:40:00.000Z');
+  });
+
+  test('refreshed_at falls back to updated_at when a snapshot predates the field', async () => {
+    // Old snapshots written before this fix have no refreshed_at — the freshness
+    // signal must still resolve (never null while a grade lock exists).
+    store['snapshot:mlb:latest'] = { updated_at: '2026-07-10T18:00:00.000Z', grades: [{ player: 'A' }] };
+    const res = await request(mountApp()).get('/api/snapshot/summary');
+    expect(res.body.refreshed_at).toBe('2026-07-10T18:00:00.000Z');
+  });
+
   test('falls back to the grades:{sport} envelope when no snapshot exists', async () => {
     store['grades:mlb'] = { updated_at: '2026-07-10T16:00:00.000Z', grades: [{ player: 'A' }, { player: 'B' }] };
     const res = await request(mountApp()).get('/api/snapshot/summary');