fix(dispatch): resolve native ingester class at call time (test monkeypatch)

The _NATIVE_INGESTERS dict captured PatreonIngester/SubscribeStarIngester at
import, so test_download_service's monkeypatch.setattr(db_mod, "PatreonIngester",
_FakeIngester) no longer affected dispatch → the fake's run() never ran →
KeyError 'campaign_id' on empty run_kwargs (integration run 1215). Replace the
dict with a _native_ingester_cls() call-time lookup that reads the module globals,
so monkeypatching the class names works again.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/app/services/download_backends.py

@@ -34,18 +34,22 @@ from .subscribestar_ingester import SubscribeStarIngester
 # deviantart) until they migrate too.
 NATIVE_INGESTER_PLATFORMS = frozenset({"patreon", "subscribestar"})
 
-# Native ingester classes keyed by platform (uniform constructor signature, so
-# _run_native_ingester / preview build whichever the source needs).
-_NATIVE_INGESTERS = {
-    "patreon": PatreonIngester,
-    "subscribestar": SubscribeStarIngester,
-}
-
 # Mirrors patreon_resolver._CAMPAIGNS_URL — surfaced in resolution-failure
 # messages so the operator sees the exact lookup endpoint that was hit.
 _CAMPAIGNS_API = "https://www.patreon.com/api/campaigns"
 
 
+def _native_ingester_cls(platform: str):
+    """The native ingester class for `platform` (uniform constructor signature).
+    A call-time lookup (not a module-level dict captured at import) so tests can
+    monkeypatch db_mod.PatreonIngester / SubscribeStarIngester and have the
+    dispatch pick up the replacement."""
+    return {
+        "patreon": PatreonIngester,
+        "subscribestar": SubscribeStarIngester,
+    }[platform]
+
+
 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."""
@@ -150,7 +154,7 @@ async def _run_native_ingester(
         if source_config.sleep_request is not None
         else max(0.5, rate_limit / 4)
     )
-    ingester = _NATIVE_INGESTERS[platform](
+    ingester = _native_ingester_cls(platform)(
         images_root=gdl.images_root,
         cookies_path=ctx["cookies_path"],
         session_factory=sync_session_factory,
@@ -209,7 +213,7 @@ async def preview_source(
                 "(cookies expired, or the creator moved/renamed?)."
             )
         }
-    ingester = _NATIVE_INGESTERS[platform](
+    ingester = _native_ingester_cls(platform)(
         images_root=images_root,
         cookies_path=cookies_path,
         session_factory=sync_session_factory,