mavcaldav: name the shared response-body read cap (V-581)

fetchEvents and listPublished both bounded their HTTP body reads at
4<<20 with no name for what the number was for. One constant,
maxResponseBody, documents the reason (cap every CalDAV response this
daemon reads) once instead of twice. No functional change.

cmd/mavcaldav/render.go

@@ -142,7 +142,7 @@ func (r *renderer) listPublished(ctx context.Context) ([]int64, error) {
 		return nil, err
 	}
 	defer resp.Body.Close()
-	raw, err := io.ReadAll(io.LimitReader(resp.Body, 4<<20))
+	raw, err := io.ReadAll(io.LimitReader(resp.Body, maxResponseBody))
 	if err != nil {
 		return nil, err
 	}