refactor: replace session key strings with consts

Define KeyPlayerID, KeyUserID, and KeyNickname in the sessions package
and use them across all handlers to avoid duplicated magic strings.

sessions/sessions.go

@@ -14,6 +14,13 @@ import (
 	"github.com/alexedwards/scs/v2"
 )
 
+// Session key names.
+const (
+	KeyPlayerID = "player_id"
+	KeyUserID   = "user_id"
+	KeyNickname = "nickname"
+)
+
 // SetupSessionManager creates a configured session manager backed by SQLite.
 // Returns the manager and a cleanup function the caller should defer.
 func SetupSessionManager(db *sql.DB) (*scs.SessionManager, func()) {
@@ -38,12 +45,12 @@ func SetupSessionManager(db *sql.DB) (*scs.SessionManager, func()) {
 // Authenticated users get their user UUID; guests get a random ID that
 // is generated and persisted on first access.
 func GetPlayerID(sm *scs.SessionManager, r *http.Request) player.ID {
-	pid := sm.GetString(r.Context(), "player_id")
+	pid := sm.GetString(r.Context(), KeyPlayerID)
 	if pid == "" {
 		pid = player.GenerateID(8)
-		sm.Put(r.Context(), "player_id", pid)
+		sm.Put(r.Context(), KeyPlayerID, pid)
 	}
-	if userID := sm.GetString(r.Context(), "user_id"); userID != "" {
+	if userID := sm.GetString(r.Context(), KeyUserID); userID != "" {
 		return player.ID(userID)
 	}
 	return player.ID(pid)
@@ -51,10 +58,10 @@ func GetPlayerID(sm *scs.SessionManager, r *http.Request) player.ID {
 
 // GetUserID returns the authenticated user's UUID, or empty string for guests.
 func GetUserID(sm *scs.SessionManager, r *http.Request) string {
-	return sm.GetString(r.Context(), "user_id")
+	return sm.GetString(r.Context(), KeyUserID)
 }
 
 // GetNickname returns the player's display name from the session.
 func GetNickname(sm *scs.SessionManager, r *http.Request) string {
-	return sm.GetString(r.Context(), "nickname")
+	return sm.GetString(r.Context(), KeyNickname)
 }