feat(downloads): native Patreon verify + uniform backend dispatch (plan #697)

The credential Verify button still ran gallery-dl --simulate for Patreon
after the cutover — testing the wrong path (and prone to the vanity
"Failed to extract campaign ID" the native resolver fixes). Wire it to the
native ingester, behind a DRY dispatch so callers never branch on platform.

- services/download_backends.py (new): the ONE place that knows which
  platforms are native vs gallery-dl. `uses_native_ingester(platform)` is
  the shared predicate; `verify_source_credential(...)` is the uniform
  probe (same (ok|None, message) contract for both backends). As a platform
  migrates, it moves into NATIVE_INGESTER_PLATFORMS here and BOTH download
  routing and verify switch together.
- PatreonClient.verify_auth(campaign_id): one authenticated /api/posts
  fetch → True (valid) / False (401/403/HTML-login) / None (drift or
  network — inconclusive, not a credential verdict).
- patreon_ingester.verify_patreon_credential(): resolve campaign id, then
  verify_auth — the verify counterpart to the download path.
- patreon_resolver.resolve_campaign_id_for_source(): extracted the
  override / id:-URL / vanity resolution into ONE helper now shared by the
  download ingester and verify (download_service no longer carries its own
  copy + regex; −`import re`).
- download_service: routes on uses_native_ingester() instead of inline
  `== "patreon"` (3 sites); uses the shared resolver.
- api/credentials: calls verify_source_credential — no platform branch.

Tests: verify_auth mapping, resolve_campaign_id_for_source (override/id:/
vanity/none), the dispatch predicate, verify_patreon_credential glue,
credentials endpoint proves Patreon uses the native path (gallery-dl verify
asserted not-called); repointed the gallery-dl verify test to subscribestar.

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

backend/app/services/patreon_ingester.py

@@ -38,6 +38,7 @@ FC runs on a plain-HTTP homelab; nothing here uses a secure-context Web API.
 
 from __future__ import annotations
 
+import asyncio
 import logging
 import time
 from collections.abc import Callable
@@ -56,6 +57,7 @@ from .patreon_client import (
     PatreonDriftError,
 )
 from .patreon_downloader import PatreonDownloader
+from .patreon_resolver import resolve_campaign_id_for_source
 
 log = logging.getLogger(__name__)
 
@@ -377,3 +379,26 @@ class PatreonIngester:
             )
             session.execute(stmt)
             session.commit()
+
+
+async def verify_patreon_credential(
+    url: str,
+    cookies_path: str | None,
+    overrides: dict | None,
+) -> tuple[bool | None, str]:
+    """Native Patreon credential probe — the verify counterpart to the ingester's
+    download path, sharing its campaign-id resolution. Resolves the campaign id
+    (override / id: URL / vanity) then does ONE authenticated `/api/posts` fetch
+    via PatreonClient.verify_auth. Returns the uniform `(ok, message)` contract
+    (True / False / None) so download_backends.verify_credential can treat it
+    interchangeably with the gallery-dl probe. No download, no DB.
+    """
+    campaign_id, _ = await resolve_campaign_id_for_source(url, cookies_path, overrides)
+    if not campaign_id:
+        return None, (
+            "Couldn't resolve the Patreon campaign id from the source URL — "
+            "can't verify (cookies expired, or the creator moved/renamed?)."
+        )
+    client = PatreonClient(cookies_path)
+    loop = asyncio.get_running_loop()
+    return await loop.run_in_executor(None, client.verify_auth, campaign_id)