feat(worker): non-forum channel-scan-skip + getChat short-circuit

For non-forum channels in runWorkerForAccount, three guards:

  1. Top-of-loop recency/backoff skip — if recently scanned with no
     pending work, or in backoff and not its turn, skip entirely.
     Bypassed when retryable SkippedPackages exist.

  2. After the SkippedPackage retry pass, a getChat short-circuit —
     if TDLib's local cache says the channel's last_message.id <= our
     effective watermark, skip the paginated searchChatMessages.

  3. End-of-scan persists lastScannedAt + lastScanFoundArchives +
     consecutiveEmptyScans via the new upsertChannelScanState helper.
     trulyIdle requires: no archives, no failures, no retryable pending.

scheduler.ts exposes getCurrentCycle() so the backoff "every Nth cycle"
modulo can be applied.

Forum-topic branch lands in the next commit.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

worker/src/scheduler.ts

@@ -19,6 +19,12 @@ let activeCyclePromise: Promise<void> | null = null;
  */
 const CYCLE_TIMEOUT_MS = (parseInt(process.env.WORKER_CYCLE_TIMEOUT_MINUTES ?? "240", 10)) * 60 * 1000;
 
+/** Read-only access to the current cycle counter for code that needs to
+ *  apply per-cycle modulo logic (e.g. the cold-channel backoff). */
+export function getCurrentCycle(): number {
+  return cycleCount;
+}
+
 /**
  * Run one ingestion cycle:
  * 1. Authenticate any PENDING accounts (triggers SMS code flow + auto-fetch channels)