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/migrate.py

@@ -13,6 +13,7 @@ from sqlalchemy import select
 from ..extensions import get_session
 from ..models import MigrationRun
 from ..tasks.migration import run_migration
+from ._responses import error_response as _bad
 
 migrate_bp = Blueprint("migrate", __name__, url_prefix="/api/migrate")
 
@@ -24,12 +25,6 @@ _VALID_KINDS = frozenset({
 _INGEST_KINDS = frozenset({"gs_ingest", "ir_ingest"})
 
 
-def _bad(error: str, *, status: int = 400, **extra):
-    body = {"error": error}
-    body.update(extra)
-    return jsonify(body), status
-
-
 def _run_to_dict(run: MigrationRun) -> dict:
     return {
         "id": run.id,