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_client.py

@@ -489,6 +489,30 @@ class PatreonClient:
                 return
             current_cursor = next_cursor
 
+    # -- verify ------------------------------------------------------------
+
+    def verify_auth(self, campaign_id: str) -> tuple[bool | None, str]:
+        """Cheap auth probe: fetch the first `/api/posts` page and report whether
+        the credential authenticated, WITHOUT downloading anything.
+
+        Returns `(ok, message)` matching the credential-verify contract:
+          - True  — authenticated (the feed returned a valid JSON:API page).
+          - False — the credential was rejected (PatreonAuthError: 401/403, or an
+                    HTML login page → cookies expired / tier insufficient).
+          - None  — inconclusive: API drift (our parser is stale, not a cred
+                    problem) or a transient network/HTTP error.
+        """
+        try:
+            response = self._fetch(campaign_id, None)
+            self._validate_response(response)
+        except PatreonAuthError as exc:
+            return False, f"Patreon rejected the credential — {exc}"
+        except PatreonDriftError as exc:
+            return None, f"Couldn't verify — Patreon's API shape changed: {exc}"
+        except PatreonAPIError as exc:
+            return None, f"Couldn't verify (network/HTTP issue): {exc}"
+        return True, "Credentials valid — the Patreon feed authenticated."
+
 
 def _dedup_by_filehash(items: list[MediaItem]) -> list[MediaItem]:
     """Drop later items sharing a filehash with an earlier one (first wins).