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

tests/test_download_service.py

@@ -9,9 +9,9 @@ from types import SimpleNamespace
 from unittest.mock import AsyncMock, MagicMock
 
 import pytest
-from sqlalchemy import select
+from sqlalchemy import func, select
 
-from backend.app.models import Artist, DownloadEvent, ImportSettings, Source
+from backend.app.models import Artist, DownloadEvent, ImageRecord, ImportSettings, Source
 from backend.app.services.credential_crypto import CredentialCrypto
 from backend.app.services.credential_service import CredentialService
 from backend.app.services.thumbnailer import Thumbnailer
@@ -152,6 +152,14 @@ async def test_download_source_attaches_written_files(
         files_downloaded=2,
         stdout=f"{f1}\n{f2}\n",
     )
+    # The ledger is marked only once the files are in: a run killed before
+    # import must leave them unmarked so the next walk imports them.
+    imported_when_marked = []
+    result.mark_seen_after_import = lambda: imported_when_marked.append(
+        db_sync.execute(
+            select(func.count(ImageRecord.id)).where(ImageRecord.path.in_([str(f1), str(f2)]))
+        ).scalar_one()
+    )
     fake_gdl = _fake_gdl_with_result(result)
 
     sync_settings = db_sync.execute(
@@ -182,6 +190,7 @@ async def test_download_source_attaches_written_files(
     assert ev.files_count == 2
     assert ev.metadata_["import_summary"]["attached"] == 2
     assert ev.metadata_["run_stats"]["downloaded_count"] == 2
+    assert imported_when_marked == [2]
 
 
 @pytest.mark.asyncio