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/posts.py

@@ -5,18 +5,11 @@ from quart import Blueprint, jsonify, request
 from ..extensions import get_session
 from ..services.post_feed_service import PostFeedService
 from ..services.source_service import KNOWN_PLATFORMS
+from ._responses import error_response as _bad
 
 posts_bp = Blueprint("posts", __name__, url_prefix="/api/posts")
 
 
-def _bad(error: str, *, status: int = 400, detail: str | None = None, **extra):
-    body = {"error": error}
-    if detail is not None:
-        body["detail"] = detail
-    body.update(extra)
-    return jsonify(body), status
-
-
 @posts_bp.route("", methods=["GET"])
 async def list_posts():
     args = request.args