feat(import-resilience L2): one-shot re-download for corrupt downloaded files

Layer 2 — remediate a corrupt file by re-fetching a fresh copy from its
source, bounded to a single attempt. Operator-requested 2026-05-28.

New backend/app/services/refetch_service.py:
- resolve_refetch_source: parse the failed file's sidecar → platform,
  derive the artist from the import path, find an ENABLED Source with a
  real feed URL for (artist, platform). Returns None for filesystem-only
  imports, missing sidecars, or `sidecar:<platform>:<slug>` synthetic
  anchors (not pollable).
- attempt_refetch: if not already refetched AND a Source resolves,
  delete the corrupt file (so gallery-dl's skip_existing re-fetches it),
  set ImportTask.refetched=True, and trigger ONE download_source
  re-check. Bounded by `refetched` so source-side corruption can't loop.

Wiring:
- Manual endpoint POST /api/import/tasks/<id>/refetch (only on 'failed'
  tasks). Returns refetch_queued / no_source / already_refetched /
  not_found / not_failed.
- Auto path in recover_interrupted_tasks: for each poison-pill row, if
  env FC_AUTO_REFETCH_CORRUPT=1, attempt_refetch (default OFF — the
  manual button is the primary path; auto is opt-in since re-fetch
  deletes a file + re-runs the downloader).
- Frontend: a cloud-refresh icon button on failed rows in ImportTaskList
  → stores.import.refetchTask → toast keyed on the result status.

Filesystem imports with no upstream return no_source — the operator's
only remediation there is replacing the file on disk, surfaced clearly
in the toast.

Tests: 404 unknown task, 400 non-failed task, no_source when
unresolvable, and the full resolvable-source path (file deleted,
refetched flag set, one download_source dispatched, second call is a
no-op). The resolvable test repoints the migration-seeded
import_settings(id=1) scan path rather than inserting a conflicting row.

backend/app/tasks/maintenance.py

@@ -1,6 +1,7 @@
 """Periodic maintenance: recover stuck import tasks, garbage-collect old finished tasks."""
 
 import logging
+import os
 import subprocess
 from datetime import UTC, datetime, timedelta
 from pathlib import Path
@@ -183,6 +184,28 @@ def recover_interrupted_tasks() -> int:
             for tid, task_type in stuck:
                 enqueue_import(tid, task_type)
 
+        # Layer-2 auto re-download (env-gated, default OFF). For each
+        # poison-pill row that resolves to a pollable Source, delete the
+        # bad file and trigger ONE source re-check to fetch a fresh
+        # copy. Bounded by ImportTask.refetched so source-side
+        # corruption can't loop. The 'failed' row stays as history; the
+        # re-downloaded file re-imports as a fresh task on the next scan.
+        if poison_ids and os.environ.get("FC_AUTO_REFETCH_CORRUPT", "0") == "1":
+            from ..models import ImportSettings
+            from ..services.refetch_service import attempt_refetch
+            import_root = Path(session.execute(
+                select(ImportSettings.import_scan_path)
+                .where(ImportSettings.id == 1)
+            ).scalar_one())
+            for pid in poison_ids:
+                ptask = session.get(ImportTask, pid)
+                if ptask is None:
+                    continue
+                try:
+                    attempt_refetch(session, ptask, import_root)
+                except Exception as exc:  # noqa: BLE001 — best-effort
+                    log.warning("auto-refetch failed for task %s: %s", pid, exc)
+
         return len(stuck) + len(poison_ids) + orphan_count