feat(pixiv): flip dispatch to the native ingester (#129 step 4)

pixiv joins NATIVE_INGESTER_PLATFORMS: download/verify/preview and the
recover/recapture UI actions now route through PixivIngester. Campaign id is
parsed straight from the source URL (numeric user id — no network resolver),
with a platform-aware resolution-failure message. auth_token now rides the
uniform adapter construction (token platforms use it, cookie platforms
accept-and-ignore), and the preview endpoint fetches/threads it. The legacy
gallery-dl pixiv path is fully removed (PLATFORM_DEFAULTS entry + the
refresh-token config branches in download/verify) per no-legacy policy;
gallery-dl keeps hentaifoundry/discord/deviantart until they migrate/retire.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01CDgx8bQS5YrGRK76v8HUnM

tests/test_download_backends.py

@@ -3,12 +3,15 @@ native ingester vs. gallery-dl. Pure, no DB."""
 
 from backend.app.services.download_backends import (
     NATIVE_INGESTER_PLATFORMS,
+    _campaign_resolution_error,
+    _native_ingester_cls,
     uses_native_ingester,
 )
+from backend.app.services.pixiv_ingester import PixivIngester
 
 
 def test_native_platforms():
-    for platform in ("patreon", "subscribestar"):
+    for platform in ("patreon", "subscribestar", "pixiv"):
         assert uses_native_ingester(platform) is True
         assert platform in NATIVE_INGESTER_PLATFORMS
 
@@ -16,9 +19,19 @@ def test_native_platforms():
 def test_gallery_dl_platforms_are_not_native():
     # The platforms still served by gallery-dl must NOT route to the native
     # ingester — guards an accidental over-broad migration.
-    for platform in ("hentaifoundry", "discord", "pixiv", "deviantart"):
+    for platform in ("hentaifoundry", "discord", "deviantart"):
         assert uses_native_ingester(platform) is False
 
 
 def test_unknown_platform_is_not_native():
     assert uses_native_ingester("nonsense") is False
+
+
+def test_pixiv_dispatches_to_its_ingester():
+    assert _native_ingester_cls("pixiv") is PixivIngester
+
+
+def test_pixiv_resolution_error_names_the_expected_url_shape():
+    msg = _campaign_resolution_error("pixiv", "https://www.pixiv.net/artworks/1")
+    assert "pixiv user id" in msg
+    assert "users/<id>" in msg