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_service.py

@@ -13,7 +13,6 @@ from __future__ import annotations
 
 import asyncio
 import logging
-import re
 from datetime import UTC, datetime
 from pathlib import Path
 from typing import Any
@@ -25,6 +24,7 @@ from sqlalchemy.orm import joinedload
 
 from ..models import Artist, DownloadEvent, Source
 from .credential_service import CredentialService
+from .download_backends import uses_native_ingester
 from .gallery_dl import (
     BACKFILL_CHUNK_SECONDS,
     BACKFILL_SKIP_VALUE,
@@ -37,28 +37,13 @@ from .gallery_dl import (
 )
 from .importer import Importer
 from .patreon_ingester import PatreonIngester
-from .patreon_resolver import resolve_campaign_id
+from .patreon_resolver import resolve_campaign_id_for_source
 from .platforms import auth_type_for
 from .scheduler_service import set_platform_cooldown
 
 log = logging.getLogger(__name__)
 
 
-# Vanity → campaign-id resolution is still needed by the native Patreon
-# ingester (phase 2 resolves the campaign id before the walk). gallery-dl's
-# reactive campaign-id retry + the `id:` effective-URL rewrite were removed at
-# the #697 cutover (Patreon no longer flows through gallery-dl).
-_PATREON_VANITY_RE = re.compile(
-    r"^https?://(?:www\.)?patreon\.com/(?:c/)?(?!id:)([^/?#]+)",
-    re.IGNORECASE,
-)
-
-
-def _extract_patreon_vanity(url: str) -> str | None:
-    m = _PATREON_VANITY_RE.match(url)
-    return m.group(1) if m else None
-
-
 class DownloadService:
     """Async orchestrator. The Celery task runs `asyncio.run(svc.download_source(N))`.
 
@@ -131,13 +116,13 @@ class DownloadService:
             skip_value: bool | str = BACKFILL_SKIP_VALUE
             source_config.timeout = BACKFILL_CHUNK_SECONDS
             pending_cursor = overrides.get("_backfill_cursor")
-            if ctx["platform"] == "patreon" and pending_cursor:
+            if uses_native_ingester(ctx["platform"]) and pending_cursor:
                 source_config.resume_cursor = pending_cursor
         else:
             skip_value = TICK_SKIP_VALUE
 
         resolved_campaign_id: str | None = None
-        if ctx["platform"] == "patreon":
+        if uses_native_ingester(ctx["platform"]):
             # Native ingester (plan #697) fully replaces gallery-dl for Patreon
             # in phase 2 — zero per-file HEADs, native cursor/resume, loud drift
             # detection. Returns a DownloadResult-shaped object so phase 3 is
@@ -198,19 +183,12 @@ class DownloadService:
         silent empty success.
         """
         overrides = ctx["config_overrides"] or {}
-        campaign_id = overrides.get("patreon_campaign_id")
-        resolved_campaign_id: str | None = None
-        if not campaign_id:
-            # A `.../id:<digits>` URL already carries the campaign id — no lookup
-            # needed (and the vanity regex deliberately excludes the id: form).
-            id_match = re.search(r"/id:(\d+)", ctx["url"])
-            if id_match:
-                campaign_id = id_match.group(1)
-            else:
-                vanity = _extract_patreon_vanity(ctx["url"])
-                if vanity:
-                    campaign_id = await resolve_campaign_id(vanity, ctx["cookies_path"])
-                    resolved_campaign_id = campaign_id
+        # Shared resolution path (override / id: URL / vanity lookup) — the same
+        # helper the credential-verify probe uses. resolved_campaign_id is
+        # non-None only when a vanity lookup ran, so phase 3 caches it.
+        campaign_id, resolved_campaign_id = await resolve_campaign_id_for_source(
+            ctx["url"], ctx["cookies_path"], overrides
+        )
 
         if not campaign_id:
             return (
@@ -540,7 +518,7 @@ class DownloadService:
         # top), so they advance only by the download archive growing.
         new_cursor = (
             parse_last_cursor(dl_result.stdout, dl_result.stderr)
-            if ctx["platform"] == "patreon" else None
+            if uses_native_ingester(ctx["platform"]) else None
         )
         advanced = bool(
             (new_cursor and new_cursor != old_cursor)