fix(tags): invalid 'copyright' kind 500 — KINDS uses real enum 'fandom'; directory 400s on bad kind

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/app/api/tags.py

@@ -56,6 +56,8 @@ async def autocomplete():
 @tags_bp.route("/tags/directory", methods=["GET"])
 async def directory():
     kind = request.args.get("kind") or None
+    if kind is not None and _coerce_kind(kind) is None:
+        return jsonify({"error": f"invalid kind {kind!r}"}), 400
     q = request.args.get("q") or None
     cursor = request.args.get("cursor") or None
     try: