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

backend/app/services/gallery_dl.py

@@ -298,8 +298,9 @@ class GalleryDLService:
     # removed at the plan-#697 cutover — it now uses the native ingester
     # (services/patreon_ingester.py), not gallery-dl.
     PLATFORM_DEFAULTS = {
-        # subscribestar removed — it's a native-ingester platform now (#71); the
-        # remaining entries are the gallery-dl platforms not yet migrated.
+        # subscribestar removed — native-ingester platform now (#71); pixiv
+        # removed likewise (#129). The remaining entries are the gallery-dl
+        # platforms not yet migrated.
         "hentaifoundry": {
             "content_types": ["all"],
             "directory": [],
@@ -315,12 +316,6 @@ class GalleryDLService:
             "reactions": False,
             "threads": True,
         },
-        "pixiv": {
-            "content_types": ["all"],
-            "directory": ["{category}"],
-            "filename": "{id}_{title[:50]}_{num:>02}.{extension}",
-            "ugoira": True,
-        },
         "deviantart": {
             "content_types": ["all"],
             "directory": [],
@@ -694,9 +689,6 @@ class GalleryDLService:
         if auth_token and platform == "discord":
             config["extractor"].setdefault("discord", {})
             config["extractor"]["discord"]["token"] = auth_token
-        if auth_token and platform == "pixiv":
-            config["extractor"].setdefault("pixiv", {})
-            config["extractor"]["pixiv"]["refresh-token"] = auth_token
 
         with tempfile.NamedTemporaryFile(
             mode="w", suffix=".json", delete=False, dir=str(self._config_dir),
@@ -882,8 +874,6 @@ class GalleryDLService:
             config["extractor"]["cookies"] = cookies_path
         if auth_token and platform == "discord":
             config["extractor"].setdefault("discord", {})["token"] = auth_token
-        if auth_token and platform == "pixiv":
-            config["extractor"].setdefault("pixiv", {})["refresh-token"] = auth_token
 
         with tempfile.NamedTemporaryFile(
             mode="w", suffix=".json", delete=False, dir=str(self._config_dir),