feat(auth): add MINDA_DEMO support with auto-seeded demo accounts and username login

internal/domain/models.go

@@ -95,6 +95,7 @@ func FormatGermanDateTime(isoStr string) string {
 type User struct {
 	ID                  string   `json:"id"`
 	Email               string   `json:"email"`
+	Username            string   `json:"username,omitempty"`
 	PasswordHash        string   `json:"-"`
 	DisplayName         string   `json:"displayName"`
 	Role                string   `json:"role"` // admin | team_admin | member