[chore/performance] Update media prune logic, add extra CLI command (#1474)

* start updating media prune stuff a wee bit

* continue prune / uncache work

* more tidying + consistency stuff

* add prune CLI command

* docs

* arg

internal/config/defaults.go

@@ -161,4 +161,6 @@ var Defaults = Configuration{
 			UserSweepFreq: time.Second * 10,
 		},
 	},
+
+	AdminMediaPruneDryRun: true,
 }