unexport things that don't need to be exported

Most of all, this lets staticcheck point out things that are unused.
Also delete an unused thing found as a result of this.

serve/dto.go

@@ -44,7 +44,7 @@ type Player struct {
 	Cuffs bool `json:"cuffs,omitempty"`
 }
 
-type GameID = uuid.UUID
+type GameID struct{ uuid.UUID }
 
 // GameStart is the JSON DTO given to each player when their game starts.
 // Observers do not receive it.