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/extension.py

@@ -20,6 +20,7 @@ from ..services.extension_service import (
     UnknownPlatformError,
 )
 from ..services.source_service import KNOWN_PLATFORMS
+from ._responses import error_response as _bad
 
 extension_bp = Blueprint("extension", __name__, url_prefix="/api/extension")
 
@@ -30,12 +31,6 @@ XPI_DIR = Path("/app/frontend/dist/extension")
 _XPI_VERSION_RE = re.compile(r"fabledcurator-(?P<version>[\w.-]+)\.xpi$")
 
 
-def _bad(error: str, *, status: int = 400, **extra):
-    body = {"error": error}
-    body.update(extra)
-    return jsonify(body), status
-
-
 async def _ext_key_required(session) -> bool:
     """Unlike /api/credentials (which accepts the browser path with no
     header), quick-add-source writes server state and must be explicitly