feat(external): download worker for file-host links (Phase 4b)

tasks/external.py drives the external_link ledger:
- fetch_external_link(link_id): atomic claim (pending/failed→downloading, so a
  duplicate enqueue no-ops), per-host Redis serialize lock (#720 pattern;
  requeue-with-countdown if busy), fetch via external_fetch into the artist
  library tree, then route each file through importer.attach_in_place via a
  synthesized sidecar so it links to the SAME post (archive→ImageRecords,
  else→PostAttachment; on-disk original removed for captured files, art stays);
  thumbnail+ML enqueue for new images; status downloaded | failed | dead with
  attempts/last_error/completed_at/duration.
- sweep_external_links(): enqueue a bounded batch of actionable links.
- recover_external_links() + prune_external_links(): recovery + retention (#89).
- per-host enable read via getattr (forward-compatible; Settings UI adds the
  columns in 4d — defaults on, rule #26).

Wiring: celery include + route (download lane) + beat (sweep 10m, recover +
prune daily); download_service phase 3 enqueues a sweep after recording links.
Integration tests: download+attach, failure, dead-letter, non-claimable, sweep.

mega still needs the MEGAcmd binary in the runtime image (Phase 4c). Refs #830.

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

backend/app/services/download_service.py

@@ -396,6 +396,13 @@ class DownloadService:
 
             await loop.run_in_executor(None, _upsert)
 
+        # Kick the off-platform file-host downloader for any links this run
+        # recorded (mega/gdrive/…). Global + idempotent (only claims pending/
+        # retryable rows); the beat sweep is the backstop. Lazy import dodges a
+        # task-module import cycle.
+        from ..tasks.external import sweep_external_links
+        sweep_external_links.delay()
+
         ev = (await self.async_session.execute(
             select(DownloadEvent).where(DownloadEvent.id == event_id)
         )).scalar_one()