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>

tests/test_download_service.py

@@ -288,7 +288,8 @@ async def test_run_patreon_ingester_resolves_vanity_and_runs(
     _artist, source = seed_artist_and_source
 
     monkeypatch.setattr(
-        dl_mod, "resolve_campaign_id", AsyncMock(return_value="4242"),
+        dl_mod, "resolve_campaign_id_for_source",
+        AsyncMock(return_value=("4242", "4242")),
     )
     run_kwargs = {}
 
@@ -335,7 +336,8 @@ async def test_run_patreon_ingester_unresolvable_fails_loud(
     _artist, source = seed_artist_and_source
 
     monkeypatch.setattr(
-        dl_mod, "resolve_campaign_id", AsyncMock(return_value=None),
+        dl_mod, "resolve_campaign_id_for_source",
+        AsyncMock(return_value=(None, None)),
     )
     svc = DownloadService(
         async_session=db, sync_session=db_sync,