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>

backend/app/services/subscribestar_client.py

@@ -27,10 +27,8 @@ plain-HTTP homelab; nothing here uses a secure-context Web API.
 
 from __future__ import annotations
 
-import http.cookiejar
 import json
 import logging
-import os
 import re
 import time
 from collections.abc import Iterator
@@ -42,16 +40,20 @@ from urllib.parse import urljoin, urlsplit
 
 import requests
 
-from ..utils.paths import filehash_from_url, safe_ext
-from .patreon_client import _MAX_429_RETRIES, _retry_after_seconds
+from ..utils.paths import filehash_from_url
+from .native_ingest_common import (
+    _MAX_429_RETRIES,
+    basename_from_url,
+    make_session,
+    retry_after_seconds,
+)
 
 log = logging.getLogger(__name__)
 
-_USER_AGENT = (
-    "Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 "
-    "(KHTML, like Gecko) Chrome/122.0.0.0 Safari/537.36"
-)
 _TIMEOUT_SECONDS = 30.0
+# Accept header for the HTML feed + JSON "load more" + the XHR marker.
+_ACCEPT = "text/html,application/xhtml+xml,application/json,*/*"
+_XHR_HEADERS = {"X-Requested-With": "XMLHttpRequest"}
 
 # A post block opens with this wrapper; we slice the page between consecutive
 # occurrences (regex can't match the balanced close of nested divs, so chunk-
@@ -122,26 +124,6 @@ class MediaItem:
     media_id: str
 
 
-def _load_session(cookies_path: str | Path | None) -> requests.Session:
-    session = requests.Session()
-    session.headers.update(
-        {
-            "User-Agent": _USER_AGENT,
-            # HTML feed + JSON "load more"; let the server pick.
-            "Accept": "text/html,application/xhtml+xml,application/json,*/*",
-            "X-Requested-With": "XMLHttpRequest",
-        }
-    )
-    if cookies_path and os.path.isfile(str(cookies_path)):
-        try:
-            jar = http.cookiejar.MozillaCookieJar(str(cookies_path))
-            jar.load(ignore_discard=True, ignore_expires=True)
-            session.cookies = jar  # type: ignore[assignment]
-        except (OSError, http.cookiejar.LoadError) as exc:
-            log.warning("Could not load SubscribeStar cookies from %s: %s", cookies_path, exc)
-    return session
-
-
 def _split_creator_url(campaign_id: str) -> tuple[str, str]:
     """`campaign_id` is the creator URL → (base, slug).
 
@@ -167,17 +149,6 @@ def _parse_ss_datetime(text: str) -> str | None:
     return None
 
 
-def _basename_from_url(url: str) -> str:
-    path = urlsplit(url).path
-    base = os.path.basename(path)
-    if base:
-        ext = safe_ext(base)
-        stem = base[: -len(Path(base).suffix)] if Path(base).suffix else base
-        stem = stem[:120] or "file"
-        return f"{stem}{ext}"
-    return "file"
-
-
 class SubscribeStarClient:
     """Synchronous SubscribeStar HTML-scrape read client. Construct with a path
     to a Netscape cookies.txt (the same file CredentialService.get_cookies_path
@@ -191,7 +162,9 @@ class SubscribeStarClient:
         max_retries: int = _MAX_429_RETRIES,
     ):
         self.cookies_path = str(cookies_path) if cookies_path else None
-        self._session = _load_session(cookies_path)
+        self._session = make_session(
+            cookies_path, accept=_ACCEPT, extra_headers=_XHR_HEADERS
+        )
         self._request_sleep = request_sleep or 0.0
         self._max_retries = max_retries
 
@@ -210,7 +183,7 @@ class SubscribeStarClient:
                 ) from exc
             if resp.status_code == 429 and attempt < self._max_retries:
                 attempt += 1
-                delay = _retry_after_seconds(resp, attempt)
+                delay = retry_after_seconds(resp, attempt)
                 log.warning(
                     "SubscribeStar 429 (%s) — backing off %.1fs (retry %d/%d)",
                     url, delay, attempt, self._max_retries,
@@ -341,7 +314,7 @@ class SubscribeStarClient:
                 url = urljoin(base + "/", rel)
                 media_id = str(it.get("id") or "")
                 name = it.get("original_filename")
-                filename = name if isinstance(name, str) and name else _basename_from_url(url)
+                filename = name if isinstance(name, str) and name else basename_from_url(url)
                 items.append(
                     MediaItem(
                         url=url,