feat(attachments): provenance payload attachments + download route

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

backend/app/api/__init__.py

@@ -17,6 +17,7 @@ def all_blueprints() -> list[Blueprint]:
     from .aliases import aliases_bp
     from .allowlist import allowlist_bp
     from .artist import artist_bp
+    from .attachments import attachments_bp
     from .gallery import gallery_bp
     from .import_admin import import_admin_bp
     from .ml_admin import ml_admin_bp
@@ -27,6 +28,7 @@ def all_blueprints() -> list[Blueprint]:
     from .tags import tags_bp
     return [
         api_bp,
+        attachments_bp,
         gallery_bp,
         provenance_bp,
         tags_bp,