feat(admin): add authenticated restart endpoint + fix admin username

- POST /api/v1/admin/restart: admin-only restart (forks new process)
- Fix: MemUserStore now uses cfg.AdminUsername instead of hardcoded 'admin'
- RestartFunc injected by main.go for clean process fork + exit

backend/services/gateway/api/auth.go

@@ -28,14 +28,16 @@ type MemUserStore struct {
 	users map[string]*UserRecord
 }
 
-// NewMemUserStore creates an in-memory user store with a default admin.
-func NewMemUserStore(adminPassword string) *MemUserStore {
+// NewMemUserStore creates an in-memory user store with the configured admin account.
+func NewMemUserStore(adminUsername, adminPassword string) *MemUserStore {
 	s := &MemUserStore{users: make(map[string]*UserRecord)}
 
 	hash, _ := auth.HashPassword(adminPassword)
-	s.users["admin"] = &UserRecord{
-		UserID: "admin", Username: "admin",
-		PasswordHash: hash, Role: "admin",
+	s.users[adminUsername] = &UserRecord{
+		UserID:       adminUsername,
+		Username:     adminUsername,
+		PasswordHash: hash,
+		Role:         "admin",
 	}
 	return s
 }