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/download_backends.py

@@ -0,0 +1,71 @@
+"""Platform → download-backend dispatch (one place that knows which platforms
+are served by the native FC ingester vs. the gallery-dl subprocess).
+
+gallery-dl wasn't built to be driven by an automated scheduler — no native
+checkpoint/resume, no structured logs, per-file HEADs that dominate wall-clock.
+The native ingester (services/patreon_ingester.py, plan #697) replaces it for
+Patreon and is the path we grow as more platforms migrate. To keep that
+migration DRY, every caller that has to behave differently per backend —
+download routing, the credential-verify probe, cursor handling — asks THIS
+module instead of testing ``platform == "patreon"`` inline. When a platform gets
+a native ingester, it moves into ``NATIVE_INGESTER_PLATFORMS`` here and both the
+download path and verify switch over together.
+
+The backend surfaces share a UNIFORM signature so a caller invokes the same
+function regardless of platform:
+  - verify_credential(...)  → (ok: bool|None, message: str)
+  - (download stays in download_service for now; uses_native_ingester() is the
+    shared predicate it routes on, so the decision lives here too.)
+"""
+
+from __future__ import annotations
+
+from pathlib import Path
+
+# Platforms whose download + verify go through the native ingester rather than
+# gallery-dl. gallery-dl still serves every other platform (subscribestar,
+# hentaifoundry, discord, pixiv, deviantart) unchanged.
+NATIVE_INGESTER_PLATFORMS = frozenset({"patreon"})
+
+
+def uses_native_ingester(platform: str) -> bool:
+    """True when `platform` is served by the native ingester (not gallery-dl).
+    The single predicate the download path and verify both route on."""
+    return platform in NATIVE_INGESTER_PLATFORMS
+
+
+async def verify_source_credential(
+    *,
+    platform: str,
+    url: str,
+    artist_slug: str,
+    config_overrides: dict | None,
+    cookies_path: str | None,
+    auth_token: str | None,
+    images_root: Path,
+) -> tuple[bool | None, str]:
+    """Uniform credential probe across backends. Returns `(ok, message)`:
+    True = authenticated, False = rejected, None = inconclusive (drift /
+    network / nothing to test). Callers don't branch on platform — they call
+    this and render the result.
+    """
+    if uses_native_ingester(platform):
+        # Native ingester platforms verify via their own lightweight auth probe
+        # (resolve campaign id + one authenticated API page). Patreon today.
+        from .patreon_ingester import verify_patreon_credential
+
+        return await verify_patreon_credential(url, cookies_path, config_overrides)
+
+    # gallery-dl platforms: --simulate one item; the extractor errors before it
+    # can list if auth is bad.
+    from .gallery_dl import GalleryDLService, SourceConfig
+
+    gdl = GalleryDLService(images_root=images_root)
+    return await gdl.verify(
+        url=url,
+        artist_slug=artist_slug,
+        platform=platform,
+        source_config=SourceConfig.from_dict(config_overrides or {}),
+        cookies_path=cookies_path,
+        auth_token=auth_token,
+    )