Add 'lastChannelRefreshedAt' to /api/v1/stats

src/invidious.cr

@@ -146,6 +146,7 @@ if config.statistics_enabled
         },
         "metadata" => {
           "updatedAt"              => Time.now.to_unix,
+          "lastChannelRefreshedAt" => PG_DB.query_one?("SELECT updated FROM channels ORDER BY updated DESC LIMIT 1", as: Time).try &.to_unix || 0,
         },
       }