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/_responses.py

@@ -0,0 +1,16 @@
+"""Shared API response helpers."""
+
+from quart import jsonify
+
+
+def error_response(
+    error: str, *, status: int = 400, detail: str | None = None, **extra,
+):
+    """JSON error body + HTTP status. `detail` is included only when given;
+    `extra` keys are merged into the body. Returns the (response, status)
+    tuple Quart expects. Imported as `_bad` by the blueprints."""
+    body = {"error": error}
+    if detail is not None:
+        body["detail"] = detail
+    body.update(extra)
+    return jsonify(body), status