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>

backend/app/services/patreon_downloader.py

@@ -124,11 +124,12 @@ def _post_dir_name(post: dict) -> str:
 class MediaOutcome:
     """Per-media result of a download_post pass.
 
-    status is one of: "downloaded", "skipped_seen", "skipped_disk", "error".
-    `path` is the final on-disk path for "downloaded" (the actual yt-dlp output
-    for video), or the path that already existed for "skipped_disk"; None for
-    "skipped_seen" and (usually) "error". `error` carries the failure reason for
-    "error", else None.
+    status is one of: "downloaded", "skipped_seen", "skipped_disk",
+    "quarantined", "error". `path` is the final on-disk path for "downloaded"
+    (the actual yt-dlp output for video), the path that already existed for
+    "skipped_disk", or the _quarantine destination for "quarantined"; None for
+    "skipped_seen" and (usually) "error". `error` carries the failure/validation
+    reason for "error"/"quarantined", else None.
     """
 
     media: object  # MediaItem (avoid importing the name for a bare annotation)
@@ -253,10 +254,13 @@ class PatreonDownloader:
             out_path = Path(out_path)
         else:
             out_path = self._fetch_get(media.url, media_path)
-            invalid = self._validate_path(out_path, artist_slug)
-            if invalid is not None:
+            reason, quarantine_dest = self._validate_path(out_path, artist_slug)
+            if reason is not None:
+                # Quarantined (corrupt/invalid) — distinct from a download error
+                # so the run can report a real files_quarantined count + paths.
                 return MediaOutcome(
-                    media=media, status="error", path=None, error=invalid
+                    media=media, status="quarantined",
+                    path=quarantine_dest, error=reason,
                 )
 
         self._write_sidecar(post, out_path)
@@ -358,23 +362,29 @@ class PatreonDownloader:
 
     # -- validation --------------------------------------------------------
 
-    def _validate_path(self, path: Path, artist_slug: str) -> str | None:
-        """Validate a freshly-written file; quarantine + return reason if bad.
+    def _validate_path(
+        self, path: Path, artist_slug: str
+    ) -> tuple[str | None, Path | None]:
+        """Validate a freshly-written file; quarantine if bad.
 
         Mirrors gallery_dl._validate_and_quarantine: fail-open for unknown
-        formats, move the corrupt file to _quarantine/<slug>/patreon, and return
-        the failure reason string (None when ok / not validatable / disabled).
+        formats, move the corrupt file to _quarantine/<slug>/patreon. Returns
+        `(reason, quarantine_dest)` when quarantined (dest is the original path if
+        the move itself failed), else `(None, None)` (ok / not validatable /
+        disabled). plan #704: the dest is surfaced so the run reports a real
+        quarantined-paths list instead of a blank count.
         """
         if not self._validate or not is_validatable(path):
-            return None
+            return None, None
         try:
             result = validate_file(path)
         except Exception as exc:
             log.warning("Validator raised on %s: %s", path, exc)
-            return None
+            return None, None
         if result.ok:
-            return None
+            return None, None
         quarantine_root = self.images_root / "_quarantine" / artist_slug / "patreon"
+        dest = path
         try:
             quarantine_root.mkdir(parents=True, exist_ok=True)
             dest = quarantine_root / path.name
@@ -385,7 +395,8 @@ class PatreonDownloader:
             shutil.move(str(path), str(dest))
         except OSError as exc:
             log.error("Failed to quarantine %s: %s. File left in place.", path, exc)
-        return result.reason or "validation failed"
+            dest = path
+        return (result.reason or "validation failed"), dest
 
     # -- sidecar -----------------------------------------------------------