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/system_backup.py

@@ -14,6 +14,7 @@ from sqlalchemy import desc, select
 
 from ..extensions import get_session
 from ..models import BackupRun, ImportSettings
+from ._responses import error_response as _bad
 
 system_backup_bp = Blueprint(
     "system_backup", __name__, url_prefix="/api/system/backup",
@@ -29,12 +30,6 @@ _BACKUP_SETTINGS_FIELDS = (
 )
 
 
-def _bad(error: str, *, status: int = 400, **extra):
-    body = {"error": error}
-    body.update(extra)
-    return jsonify(body), status
-
-
 def _row_to_dict(r: BackupRun) -> dict:
     return {
         "id": r.id,