feat(model): add Status field and constants to User and File

Add Status field (gorm, indexed, default active) to both User and File models.

Add User status constants: StatusActive, StatusAdminDeleted.

Add File status constant: StatusUserDeleted.

Add tests verifying Status field is excluded from JSON serialization.

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

internal/model/user.go

@@ -11,6 +11,13 @@ type User struct {
 	Email        string    `gorm:"uniqueIndex;type:varchar(255);not null" json:"email"`
 	PasswordHash string    `gorm:"type:varchar(255);not null" json:"-"`
 	IsAdmin      bool      `gorm:"default:false" json:"is_admin"`
+	Status       string    `gorm:"type:varchar(32);not null;default:active;index" json:"-"`
 	CreatedAt    time.Time `json:"created_at"`
 	UpdatedAt    time.Time `json:"updated_at"`
 }
+
+// User status constants.
+const (
+	StatusActive       = "active"
+	StatusAdminDeleted = "admin_deleted"
+)