refactor(native-ingest): extract native_ingest_common + BaseNativeDownloader (#899 DRY 1/3)

DRY pass commit 1 (process #594). Consolidate the helpers + download plumbing
the Patreon and SubscribeStar adapters had duplicated (SubscribeStar was
importing patreon privates — wrong owner). New backend/app/services/
native_ingest_common.py is the neutral home for:
- make_session (was _load_session ×2), retry_after_seconds + 429 constants,
  sanitize_segment, basename_from_url, post_dir_name, MediaOutcome /
  PostRecordOutcome.
- BaseNativeDownloader: the shared streaming GET (transient-retry + Range-resume)
  and validation/quarantine. Patreon + SubscribeStar downloaders now subclass it;
  each keeps only what differs (Patreon's Mux/yt-dlp video branch + detail-fetch
  enrichment; SubscribeStar nothing extra). Behavior preserved exactly; the
  divergence-bug risk (a fix to one _fetch_to_file not reaching the other) is gone.
- Folds in #899 L2: a quarantine now log.warning's path+reason (was counted only).

post_dir_name merges both date handlers (accepts trailing-Z and pre-parsed ISO).
Tests repointed to the single source at every consumer (rule 93 / §8b parity):
patreon_client/downloader, subscribestar_native. Exception-trio consolidation +
base _failure_result (2/3) and the remaining ingest_core logging (3/3) follow.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tests/test_subscribestar_native.py

@@ -14,6 +14,7 @@ from pathlib import Path
 import requests
 
 from backend.app.services.gallery_dl import DownloadResult, ErrorType
+from backend.app.services.native_ingest_common import post_dir_name
 from backend.app.services.subscribestar_client import (
     MediaItem,
     SubscribeStarAPIError,
@@ -22,10 +23,7 @@ from backend.app.services.subscribestar_client import (
     SubscribeStarDriftError,
     _parse_ss_datetime,
 )
-from backend.app.services.subscribestar_downloader import (
-    SubscribeStarDownloader,
-    _post_dir_name,
-)
+from backend.app.services.subscribestar_downloader import SubscribeStarDownloader
 from backend.app.services.subscribestar_ingester import (
     SubscribeStarIngester,
     _ledger_key,
@@ -210,7 +208,7 @@ def _downloader(tmp_path: Path, session=None, validate=True):
 def test_post_dir_name_empty_title_matches_gallery_dl():
     # SubscribeStar has no title → "<date>_<id>_" (matches gallery-dl's layout so
     # existing downloads dedup on disk at cutover).
-    assert _post_dir_name(_post("111")) == "2026-05-01_111_"
+    assert post_dir_name(_post("111")) == "2026-05-01_111_"
 
 
 def test_download_post_layout_and_outcomes(tmp_path):