feat(db): add scan-state columns to AccountChannelMap + TopicProgress

Three new fields on each table:
  - lastScannedAt          — when the worker last touched this scope
  - lastScanFoundArchives  — true if last scan had archives OR pending
                             retryables; tracks "work might need revisit"
  - consecutiveEmptyScans  — counter for cold-channel backoff

Schema change only. Worker logic in follow-up commits. Migration is a
metadata-only ALTER (NOT NULL with default) so it runs in ms even on
21k+ Package rows.

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

prisma/migrations/20260526000000_channel_scan_state/migration.sql

@@ -0,0 +1,11 @@
+-- AlterTable: per-channel scan-state columns
+ALTER TABLE "account_channel_map"
+  ADD COLUMN "lastScannedAt" TIMESTAMP(3),
+  ADD COLUMN "lastScanFoundArchives" BOOLEAN NOT NULL DEFAULT false,
+  ADD COLUMN "consecutiveEmptyScans" INTEGER NOT NULL DEFAULT 0;
+
+-- AlterTable: per-topic scan-state columns (forum channels)
+ALTER TABLE "topic_progress"
+  ADD COLUMN "lastScannedAt" TIMESTAMP(3),
+  ADD COLUMN "lastScanFoundArchives" BOOLEAN NOT NULL DEFAULT false,
+  ADD COLUMN "consecutiveEmptyScans" INTEGER NOT NULL DEFAULT 0;

prisma/schema.prisma

@@ -450,6 +450,17 @@ model AccountChannelMap {
   channelId              String
   role                   ChannelRole @default(READER)
   lastProcessedMessageId BigInt?
+  /// When this channel was last scanned (any reason, including skipped scans
+  /// that bumped the timestamp). Used by the recency-skip guard.
+  lastScannedAt          DateTime?
+  /// True if the last scan found archives OR left retryable SkippedPackages
+  /// pending. Tracks "this channel has work I might need to revisit" — not
+  /// just "I uploaded something this cycle".
+  lastScanFoundArchives  Boolean     @default(false)
+  /// Number of consecutive cycles where this channel was trulyIdle (no
+  /// archives, no failures, no retryables). Drives the backoff that lets
+  /// cold channels skip cycles entirely.
+  consecutiveEmptyScans  Int         @default(0)
   createdAt              DateTime    @default(now())
 
   account        TelegramAccount @relation(fields: [accountId], references: [id], onDelete: Cascade)
@@ -587,6 +598,14 @@ model TopicProgress {
   topicId                BigInt
   topicName              String?
   lastProcessedMessageId BigInt?
+  /// When this topic was last scanned (any reason). Used by recency-skip.
+  lastScannedAt          DateTime?
+  /// True if the last scan found archives OR has retryable SkippedPackages
+  /// pending for this topic. See AccountChannelMap doc for details.
+  lastScanFoundArchives  Boolean   @default(false)
+  /// Number of consecutive cycles where this topic was trulyIdle. Drives
+  /// backoff for cold topics.
+  consecutiveEmptyScans  Int       @default(0)
 
   accountChannelMap AccountChannelMap @relation(fields: [accountChannelMapId], references: [id], onDelete: Cascade)