feat(patreon): structured ingester results + quarantine surfacing — #704 step 1

The native ingester faked gallery-dl stdout (`Cursor:` lines, summary) and
phase 3 regex-scraped it back — so Patreon run-stats were mostly zero and
quarantine stats blank. We own the ingester, so it now RETURNS structured
data and phase 3 reads it directly.

- DownloadResult gains run_stats/cursor/posts_processed (None/0 on the
  gallery-dl path, which keeps the text route).
- Ingester builds real run_stats from per-media outcome counts, sets the
  checkpoint cursor structurally (no fake `Cursor:` stdout), and counts
  posts processed. download_service phase 3 uses dl_result.run_stats when
  present; the backfill lifecycle + TIMEOUT→PARTIAL block checkpoint
  dl_result.cursor instead of parse_last_cursor(stdout).
- #4 quarantine: PatreonDownloader reports a distinct "quarantined"
  MediaOutcome (with the _quarantine dest); the ingester surfaces a real
  files_quarantined + quarantined_paths + run_stats.quarantined_count
  (was hardcoded 0). Quarantined media isn't written or marked seen.
- Cleanup: parse_last_cursor + _CURSOR_RE (and the now-unused `import re`)
  removed from gallery_dl — the structured cursor replaced the scrape.

Tests: ingester result carries real run_stats/cursor/posts_processed +
quarantine counts; downloader quarantines an invalid file as "quarantined";
backfill cursor tests pass cursor= structurally; dropped the
parse_last_cursor tests.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/test_patreon_downloader.py

@@ -165,6 +165,22 @@ def test_skip_seen(tmp_path):
     assert not (tmp_path / "artist-x").exists()
 
 
+def test_invalid_file_is_quarantined(tmp_path):
+    """plan #704 (#4): a downloaded file that fails validation is moved to
+    _quarantine and reported as a 'quarantined' outcome carrying that path —
+    distinct from a download 'error'."""
+    bad_url = "https://cdn.patreon.com/bad.png"
+    session = _FakeSession({bad_url: b"this is not a valid PNG"})
+    dl = _downloader(tmp_path, session=session, validate=True)
+    outcomes = dl.download_post(
+        _post(), [_img("bad.png", url=bad_url)], "artist-x"
+    )
+    assert outcomes[0].status == "quarantined"
+    assert outcomes[0].path is not None
+    assert "_quarantine" in str(outcomes[0].path)
+    assert outcomes[0].path.is_file()
+
+
 def test_skip_disk(tmp_path):
     dl = _downloader(tmp_path)
     post_dir = tmp_path / "artist-x" / "patreon" / "2026-05-01_1001_My Post Title"