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/ingest_core.py

@@ -32,7 +32,7 @@ from collections.abc import Callable
 from sqlalchemy import delete, func, select, text
 from sqlalchemy.dialects.postgresql import insert as pg_insert
 
-from .gallery_dl import DownloadResult, ErrorType
+from .gallery_dl import DownloadResult, ErrorType, make_run_stats
 
 log = logging.getLogger(__name__)
 
@@ -153,16 +153,14 @@ class Ingester:
                 duration_seconds=time.monotonic() - start,
                 cursor=emitted_cursor,
                 posts_processed=posts_processed,
-                run_stats={
-                    "exit_code": return_code,
-                    "downloaded_count": downloaded,
-                    "skipped_count": skipped_count,
-                    "per_item_failures": errors,
-                    "warning_count": 0,
-                    "tier_gated_count": 0,
-                    "quarantined_count": quarantined,
-                    "dead_lettered_count": dead_lettered,
-                },
+                run_stats=make_run_stats(
+                    exit_code=return_code,
+                    downloaded_count=downloaded,
+                    skipped_count=skipped_count,
+                    per_item_failures=errors,
+                    quarantined_count=quarantined,
+                    dead_lettered_count=dead_lettered,
+                ),
             )
 
         try: