Restore --reset-admin-password flag for CLI admin password reset

backend/internal/db/imc_users.go

@@ -68,6 +68,17 @@ func (d *DB) DeleteImcUser(id uint) error {
 	return d.Delete(&ImcUser{}, id).Error // Delete by primary key
 }
 
+// UpsertAdminUser creates the admin user if it doesn't exist, or updates the password if it does.
+func (d *DB) UpsertAdminUser(username, passwordHash string) error {
+	var user ImcUser
+	err := d.Where("username = ? AND role = ?", username, "admin").First(&user).Error
+	if err == nil {
+		return d.Model(&ImcUser{}).Where("id = ?", user.ID).Update("password_hash", passwordHash).Error
+	}
+	_, err = d.CreateImcUser(username, passwordHash, "admin")
+	return err
+}
+
 // =============================================================================
 // Domain Access Control
 // These functions manage which domains users can access.