add field to hold timestamp of cached result

Will help to debug certain things to be able to more selectively flush the cache

backend/tables/travel_time_cache.sql

@@ -2,6 +2,7 @@ CREATE TABLE nwessel.cached_travel_times (
     uri_string text CHECK(uri_string ~ '^\/\d+\/\d+\/\d{1,2}\/\d{1,2}\/\d{4}-\d{2}-\d{2}\/\d{4}-\d{2}-\d{2}\/(true|false)\/[1-7]{1,7}$'),
     commit_hash text,
     results jsonb NOT NULL,
+    first_requested timestamptz DEFAULT NOW(),
     PRIMARY KEY (uri_string, commit_hash)
 );