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_patreon_ingester.py

@@ -370,3 +370,32 @@ def test_ledger_key_video_has_no_filehash():
         kind="postfile", filehash=None, post_id="p9",
     )
     assert _ledger_key(video) == "p9:clip.mp4"
+
+
+# --- verify_patreon_credential (the verify counterpart, plan #697) ---------
+
+
+@pytest.mark.asyncio
+async def test_verify_patreon_credential_unresolvable_is_inconclusive():
+    from backend.app.services.patreon_ingester import verify_patreon_credential
+
+    # No campaign id resolvable from a non-Patreon URL → can't test → None.
+    ok, msg = await verify_patreon_credential("not-a-patreon-url", None, {})
+    assert ok is None
+    assert "campaign id" in msg.lower()
+
+
+@pytest.mark.asyncio
+async def test_verify_patreon_credential_delegates_to_client(monkeypatch):
+    import backend.app.services.patreon_ingester as mod
+
+    async def _fake_resolve(url, cookies_path, overrides):
+        return "123", None
+    monkeypatch.setattr(mod, "resolve_campaign_id_for_source", _fake_resolve)
+    monkeypatch.setattr(
+        mod.PatreonClient, "verify_auth",
+        lambda self, campaign_id: (True, f"ok:{campaign_id}"),
+    )
+    ok, msg = await mod.verify_patreon_credential("https://patreon.com/x", None, {})
+    assert ok is True
+    assert msg == "ok:123"