fc5: /api/migrate blueprint — multipart upload for ingest kinds + JSON for the rest + apply-without-backup guard

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/app/api/__init__.py

@@ -23,6 +23,7 @@ def all_blueprints() -> list[Blueprint]:
     from .downloads import downloads_bp
     from .gallery import gallery_bp
     from .import_admin import import_admin_bp
+    from .migrate import migrate_bp
     from .ml_admin import ml_admin_bp
     from .platforms import platforms_bp
     from .posts import posts_bp
@@ -43,6 +44,7 @@ def all_blueprints() -> list[Blueprint]:
         showcase_bp,
         settings_bp,
         import_admin_bp,
+        migrate_bp,
         suggestions_bp,
         allowlist_bp,
         aliases_bp,