refactor(dry-B1): consolidate duplicated _bad error helper into api/_responses.py

8 blueprints each defined an identical _bad() (two variants: with/without
detail). Extracted error_response() into api/_responses.py; each blueprint
now imports it `as _bad` so call sites are unchanged. The detail-aware
canonical subsumes both variants. Left settings.py's distinct _bad_int and
the inline jsonify error sites (not duplicated helpers).

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

backend/app/api/admin.py

@@ -24,16 +24,11 @@ from sqlalchemy import select
 from ..extensions import get_session
 from ..models import Artist
 from ..services.cleanup_service import project_artist_cascade, project_bulk_image_delete
+from ._responses import error_response as _bad
 
 admin_bp = Blueprint("admin", __name__, url_prefix="/api/admin")
 
 
-def _bad(error: str, *, status: int = 400, **extra):
-    body = {"error": error}
-    body.update(extra)
-    return jsonify(body), status
-
-
 def _bulk_image_confirm_token(image_ids: list[int]) -> str:
     """Stable 8-hex token derived from the sorted id list. Mutates
     when the selection changes; stays the same across modal opens of