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/models/external_link.py

@@ -86,6 +86,11 @@ class ExternalLink(Base):
     created_at: Mapped[datetime] = mapped_column(
         DateTime(timezone=True), nullable=False, server_default=func.now()
     )
+    # When the current fetch claimed the link ('downloading'). The sweep uses it
+    # to recover a link whose worker died mid-fetch.
+    started_at: Mapped[datetime | None] = mapped_column(
+        DateTime(timezone=True), nullable=True
+    )
     completed_at: Mapped[datetime | None] = mapped_column(
         DateTime(timezone=True), nullable=True
     )