feat(db): add fetchEnabled flag to TopicProgress

Additive Postgres migration (default true) that backs the per-topic
fetch enable/disable toggle. Existing rows backfill to enabled, so
current behaviour is unchanged. Migration applied to the live DB via
CI prisma migrate deploy (deferred, per environment).

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

prisma/schema.prisma

@@ -597,6 +597,11 @@ model TopicProgress {
   accountChannelMapId    String
   topicId                BigInt
   topicName              String?
+  /// User-controlled fetch toggle. When false, the worker skips this topic
+  /// (no scanning, no fetching/transfer). Defaults true so every existing and
+  /// newly-discovered topic is fetched unless explicitly disabled. Disabling
+  /// is non-destructive — already-fetched packages are kept.
+  fetchEnabled           Boolean   @default(true)
   lastProcessedMessageId BigInt?
   /// When this topic was last scanned (any reason). Used by recency-skip.
   lastScannedAt          DateTime?