refactor: replace hardcoded NATS subjects with typed helpers

Add GameSubject/ChatSubject helpers to connect4 and snake packages,
eliminating magic string concatenation from handlers and main.go.

connect4/types.go

@@ -6,6 +6,12 @@ import (
 	"github.com/ryanhamamura/games/player"
 )
 
+// NATS subject helpers.
+const SubjectPrefix = "connect4"
+
+func GameSubject(gameID string) string { return SubjectPrefix + "." + gameID }
+func ChatSubject(gameID string) string { return SubjectPrefix + ".chat." + gameID }
+
 type Player struct {
 	ID       player.ID
 	UserID   *string // UUID for authenticated users, nil for guests