fix: a download is marked seen only after it is imported

A run killed between download and import left its files on disk and in the
seen-ledger but never in the library, and every later walk trusted the
ledger. TamadaHeijun's 12PCG post lost 7 of 13 images this way (stranded
run 90402), which read as the duplicates filter.

The ingester now hands phase 3 a mark_seen_after_import hook, called after
the import loop. A file on disk with no ImageRecord at its path is fed to
import, not reconciled into the ledger. Recapture reaches files already
orphaned, because it looks past the ledger to the disk.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LVjrnpQjRgHdvq95rASoiR

backend/app/services/download_service.py

@@ -423,6 +423,13 @@ class DownloadService:
 
             await loop.run_in_executor(None, _upsert)
 
+        # Only now is it safe to call this walk's media seen: every file above
+        # has been through the importer. Had the run died before here they stay
+        # unmarked, and the next walk imports them from disk (ingest_core).
+        mark_seen = getattr(dl_result, "mark_seen_after_import", None)
+        if mark_seen is not None:
+            await loop.run_in_executor(None, mark_seen)
+
         # #830 recapture: backfill source_filehash on EXISTING on-disk images so
         # their post-body inline <img src=CDN> remaps to the local copy. A
         # SEPARATE non-deleting channel (NOT the import list — that would unlink