refactor(downloads): DRY the ingester/gallery-dl seam — A1–A4 (plan #707)

Consolidates duplication that owning the native ingester left against the still-
live gallery-dl path, and fixes a parity gap the duplication hid.

A1 — shared quarantine: extract file_validator.quarantine_file (move to
_quarantine/<slug>/<platform> + write the .quarantine.json provenance sidecar).
gallery_dl._validate_and_quarantine and patreon_downloader._validate_path both
call it. PARITY FIX: the native path now writes the provenance sidecar it
previously skipped — threads the media url through for source_url.

A2 — make_run_stats(**counts) factory in gallery_dl for the canonical run_stats
key set; gallery_dl._compute_run_stats and ingest_core both build through it so
the shape can't drift (gallery-dl path gains a benign dead_lettered_count=0).

A3 — one safe_ext in utils/paths.py; importer._safe_ext (thin wrapper, kept for
the Path call sites + memory pointer) and patreon_client both use it. Closes the
double-impl of the URL-encoded-basename ext gotcha.

A4 — promote gallery_dl._truncate_log/_extract_errors_warnings to module-level
truncate_log/extract_errors_warnings; download_service calls them directly
instead of reaching through self.gdl for native-result log shaping. The
staticmethods stay as thin delegators for existing callers/tests.

Behavior-preserving except the A1 sidecar parity fix. Test: native quarantine
writes a .quarantine.json (test_patreon_downloader).

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

backend/app/services/download_service.py

@@ -33,6 +33,8 @@ from .gallery_dl import (
     ErrorType,
     GalleryDLService,
     SourceConfig,
+    extract_errors_warnings,
+    truncate_log,
 )
 from .importer import Importer
 from .patreon_ingester import PatreonIngester
@@ -450,7 +452,7 @@ class DownloadService:
                 dl_result.return_code, dl_result.stdout, dl_result.stderr
             )
         run_stats["quarantined_count"] = dl_result.files_quarantined
-        stderr_summary = self.gdl._extract_errors_warnings(dl_result.stderr)
+        stderr_summary = extract_errors_warnings(dl_result.stderr)
 
         # Plan #544: PARTIAL means the run downloaded ≥1 file but the
         # subprocess didn't finish in budget (typically wall-clock timeout
@@ -470,8 +472,8 @@ class DownloadService:
         ev.metadata_ = {
             "run_stats": run_stats,
             "error_type": dl_result.error_type.value if dl_result.error_type else None,
-            "stdout": self.gdl._truncate_log(dl_result.stdout) or None,
-            "stderr": self.gdl._truncate_log(dl_result.stderr) or None,
+            "stdout": truncate_log(dl_result.stdout) or None,
+            "stderr": truncate_log(dl_result.stderr) or None,
             "stderr_errors_warnings": stderr_summary or None,
             "duration_seconds": dl_result.duration_seconds,
             "quarantined_paths": dl_result.quarantined_paths or None,