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_patreon_downloader.py

@@ -14,13 +14,14 @@ import pytest
 import requests
 
 from backend.app.services import patreon_downloader as pd_mod
-from backend.app.services.patreon_client import MediaItem
-from backend.app.services.patreon_downloader import (
+from backend.app.services.native_ingest_common import (
     MediaOutcome,
-    PatreonDownloader,
     PostRecordOutcome,
-    _sanitize,
+    post_dir_name,
+    sanitize_segment,
 )
+from backend.app.services.patreon_client import MediaItem
+from backend.app.services.patreon_downloader import PatreonDownloader
 from backend.app.utils.sidecar import find_sidecar
 
 # Minimal valid PNG so the file_validator passes on the happy path.
@@ -414,10 +415,10 @@ def test_validation_off_keeps_bytes(tmp_path):
 
 
 def test_sanitize_helper():
-    assert _sanitize('a/b') == "a_b"
-    assert _sanitize('x<>:"|?*y') == "x_______y"
-    assert _sanitize("trail.  ") == "trail"
-    assert _sanitize("") == "_"
+    assert sanitize_segment('a/b') == "a_b"
+    assert sanitize_segment('x<>:"|?*y') == "x_______y"
+    assert sanitize_segment("trail.  ") == "trail"
+    assert sanitize_segment("") == "_"
 
 
 def test_media_outcome_shape():
@@ -613,7 +614,7 @@ def test_media_sidecar_is_minimal_post_first(tmp_path):
     post["attributes"]["content"] = ""  # even an empty feed body isn't fetched here
     dl.download_post(post, [_img("a.png"), _img("b.png")], "artist-x")
 
-    post_dir = tmp_path / "artist-x" / "patreon" / pd_mod._post_dir_name(post)
+    post_dir = tmp_path / "artist-x" / "patreon" / post_dir_name(post)
     sc = find_sidecar(post_dir / "01_a.png")
     assert sc is not None
     data = json.loads(sc.read_text())