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_test.go

@@ -0,0 +1,29 @@
+package model
+
+import (
+	"encoding/json"
+	"testing"
+)
+
+func TestUserJSONOmitsStatusField(t *testing.T) {
+	u := User{
+		ID:       "user-1",
+		Username: "alice",
+		Email:    "alice@example.com",
+		Status:   StatusActive,
+	}
+
+	raw, err := json.Marshal(u)
+	if err != nil {
+		t.Fatalf("json.Marshal: %v", err)
+	}
+
+	var m map[string]interface{}
+	if err := json.Unmarshal(raw, &m); err != nil {
+		t.Fatalf("json.Unmarshal: %v", err)
+	}
+
+	if _, ok := m["status"]; ok {
+		t.Error("Status field should not appear in JSON output")
+	}
+}