fix(coordinator): stop masking 501 as an internal error

Unimplemented endpoints answered {"error":"internal error"} because the
detail-hiding branch covered everything >= 500, and 501 falls in that range.
That is actively misleading: it sent one debugging session looking for a
database fault when the endpoint simply had no implementation yet.

501 now says "not implemented". It leaks nothing — the client already knows the
status code — while genuine 5xx responses keep hiding their details behind a
request id.

coordinator/internal/transport/http/errors.go

@@ -44,6 +44,14 @@ func (s *Server) writeError(w http.ResponseWriter, r *http.Request, err error) {
 		status = http.StatusNotImplemented
 	}
 
+	// 501 says "this endpoint has no implementation yet" — that leaks nothing and
+	// is far more useful than a generic failure, which sent one debugging session
+	// hunting a database problem that did not exist.
+	if status == http.StatusNotImplemented {
+		writeJSON(w, status, errorResponse{Error: "not implemented", RequestID: reqID})
+		return
+	}
+
 	if status >= 500 {
 		// Never echo an internal error: it can carry table names, query
 		// fragments, and values. The request ID is the bridge to the logs.