fix(main): R9 — guard the yt-dlp update throttle against a backward clock

shouldAutoCheckYtdlp compared (now - lastCheck) >= interval; a backward system-
clock correction leaves lastCheck in the future so the difference is negative and
the daily check never comes due again. Treat a future lastCheck as skewed and run
the check. The renderer scheduled-item promoter is inherently fire-once (promotion
flips the item out of 'saved'), so it can't double-fire on a backward jump — a
forward jump firing early is documented and accepted as minor.

Unit-tested. typecheck + 263 tests + eslint + prettier green.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

test/ytdlpPolicy.test.ts

@@ -31,4 +31,11 @@ describe('shouldAutoCheckYtdlp', () => {
     expect(shouldAutoCheckYtdlp(true, NOW - 30 * 60_000, NOW, hour)).toBe(false)
     expect(shouldAutoCheckYtdlp(true, NOW - hour, NOW, hour)).toBe(true)
   })
+
+  it('checks when the clock ran backward (lastCheck is in the future, R9)', () => {
+    // A backward clock correction leaves lastCheck > now → (now - lastCheck) is
+    // negative; without the guard the check would be skipped forever.
+    expect(shouldAutoCheckYtdlp(true, NOW + AUTO_UPDATE_INTERVAL_MS, NOW)).toBe(true)
+    expect(shouldAutoCheckYtdlp(true, NOW + 1, NOW)).toBe(true)
+  })
 })