fix: downloads stop leaving files on disk that the library doesn't use

- An archive whose images were extracted was kept at its download path
  as well as in the attachments store. Both download paths now remove
  the downloaded copy.
- A downloaded file the import filters reject (too small, transparent,
  single colour, unreadable) was left on disk with no record. A later
  walk found it, imported it again and rejected it again, every time.
  Rejected files are now removed; the seen ledger keeps them from being
  downloaded again.
- Both paths share one policy, Importer.discard_after_attach, which
  never removes a file an ImageRecord points at.
- An external-link fetch that fails, or whose worker was killed, left
  partial files (megatools' .megatmp.*) in the link's dir, and a killed
  fetch left its link 'downloading' for good. A fetch now clears
  unrecorded leftovers before starting and after failing. The sweep
  returns a link to retry once it has been 'downloading' past the
  task's hard limit, using a new external_link.started_at column
  (migration 0117).
- The orphaned temp-file sweep now recognises .megatmp.* as well as
  .part and .partial.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

backend/app/services/download_service.py

@@ -369,11 +369,8 @@ class DownloadService:
                     if do_embed:
                         embed_image.delay(img_id)
             elif result.status == "attached":
-                # Non-media or extracted archive captured as PostAttachment
-                # (FC-2d-iii). The canonical copy lives in the attachments
-                # store; the original download path is now redundant —
-                # mirror duplicate_hash cleanup so we don't keep two copies.
-                # Operator-flagged 2026-06-02 (Lustria OST zip).
+                # Non-media, or an archive with no new images, captured as a
+                # PostAttachment; the attachments store holds the copy.
                 import_summary["attached"] += 1
                 # An archive captured WITHOUT extracting any image carries a
                 # reason on result.error — record it so the event explains the
@@ -390,35 +387,13 @@ class DownloadService:
                     bytes_downloaded += path.stat().st_size  # noqa: ASYNC240
                 except OSError:
                     pass
-                try:
-                    path.unlink(missing_ok=True)  # noqa: ASYNC240
-                except OSError:
-                    pass
-            elif result.status == "skipped" and result.skip_reason and result.skip_reason.value in (
-                "duplicate_hash", "duplicate_phash",
-            ):
-                import_summary["skipped"] += 1
-                try:
-                    path.unlink(missing_ok=True)  # noqa: ASYNC240
-                except OSError:
-                    pass
             elif result.status == "skipped":
-                # Soft skip (too_small, too_transparent, invalid_image) —
-                # the file just didn't qualify, not a download/ingest
-                # failure. Don't flag the run as error; the file stays
-                # on disk for operator inspection.
+                # A duplicate, or a file the import filters rejected. Neither
+                # is a download failure, so the run is not flagged.
                 import_summary["skipped"] += 1
             elif result.status == "failed":
-                # Hard failure (today only: archive probe crash/timeout).
-                # The original archive sits in /images/ as an orphan; the
-                # filesystem scanner would re-import and re-crash on the
-                # same file, so delete the source file and surface the
-                # error in import_summary. Audit 2026-06-02.
+                # Hard failure: an archive probe crashed or timed out.
                 import_summary["errors"] += 1
-                try:
-                    path.unlink(missing_ok=True)  # noqa: ASYNC240
-                except OSError:
-                    pass
             elif result.status == "refreshed":
                 # Currently unreachable from attach_in_place (the download
                 # path never runs in deep=True mode), but the importer's
@@ -427,6 +402,10 @@ class DownloadService:
                 import_summary["attached"] += 1
             else:
                 import_summary["errors"] += 1
+            # Remove the download when the library no longer needs it at this
+            # path: an attachment or archive now in the store, a duplicate, a
+            # rejected file. One policy for every download path.
+            self.importer.discard_after_attach(path, result)
 
         # Post-only records: media-less posts (pure-text) the native ingester
         # captured so the artist archive is complete. Upsert each (keyed on