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_resolver.py

@@ -2,7 +2,10 @@ from unittest.mock import MagicMock, patch
 
 import pytest
 
-from backend.app.services.patreon_resolver import resolve_campaign_id
+from backend.app.services.patreon_resolver import (
+    resolve_campaign_id,
+    resolve_campaign_id_for_source,
+)
 
 
 @pytest.mark.asyncio
@@ -86,3 +89,48 @@ async def test_loads_cookies_file_when_present(tmp_path):
     assert result == "9"
     _, kwargs = mock_get.call_args
     assert kwargs.get("cookies") is not None
+
+
+# -- resolve_campaign_id_for_source (shared by download + verify) ----------
+
+
+@pytest.mark.asyncio
+async def test_for_source_uses_cached_override_without_lookup():
+    with patch("backend.app.services.patreon_resolver.requests.get") as mock_get:
+        cid, resolved = await resolve_campaign_id_for_source(
+            "https://patreon.com/alice", None, {"patreon_campaign_id": "999"},
+        )
+    assert cid == "999"
+    assert resolved is None  # cached → no newly-resolved id to cache
+    mock_get.assert_not_called()
+
+
+@pytest.mark.asyncio
+async def test_for_source_extracts_id_url_without_lookup():
+    with patch("backend.app.services.patreon_resolver.requests.get") as mock_get:
+        cid, resolved = await resolve_campaign_id_for_source(
+            "https://www.patreon.com/id:4242", None, {},
+        )
+    assert cid == "4242"
+    assert resolved is None
+    mock_get.assert_not_called()
+
+
+@pytest.mark.asyncio
+async def test_for_source_resolves_vanity_and_reports_it():
+    fake = MagicMock()
+    fake.status_code = 200
+    fake.json.return_value = {"data": [{"id": "777", "type": "campaign"}]}
+    with patch("backend.app.services.patreon_resolver.requests.get", return_value=fake):
+        cid, resolved = await resolve_campaign_id_for_source(
+            "https://patreon.com/alice", None, {},
+        )
+    assert cid == "777"
+    assert resolved == "777"  # newly resolved → caller caches it
+
+
+@pytest.mark.asyncio
+async def test_for_source_unresolvable_returns_none():
+    cid, resolved = await resolve_campaign_id_for_source("not-a-patreon-url", None, {})
+    assert cid is None
+    assert resolved is None