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/file_test.go

@@ -0,0 +1,49 @@
+package model
+
+import (
+	"encoding/json"
+	"testing"
+	"time"
+)
+
+func TestFile_StatusExcludedFromJSON(t *testing.T) {
+	f := &File{
+		ID:        "1",
+		UserID:    "u1",
+		ParentID:  nil,
+		Name:      "test.txt",
+		Size:      100,
+		MimeType:  "text/plain",
+		Status:    StatusActive,
+		IsDir:     false,
+		CreatedAt: time.Now(),
+		UpdatedAt: time.Now(),
+	}
+
+	data, err := json.Marshal(f)
+	if err != nil {
+		t.Fatalf("json.Marshal: %v", err)
+	}
+
+	var result map[string]any
+	if err := json.Unmarshal(data, &result); err != nil {
+		t.Fatalf("json.Unmarshal: %v", err)
+	}
+
+	if _, ok := result["status"]; ok {
+		t.Errorf("Status field should be excluded from JSON (json:\"-\"), but it appeared in output")
+	}
+
+	if _, ok := result["hash"]; ok {
+		t.Errorf("Hash field should be excluded from JSON (json:\"-\"), but it appeared in output")
+	}
+}
+
+func TestStatusConstants(t *testing.T) {
+	if StatusActive != "active" {
+		t.Errorf("StatusActive = %q, want \"active\"", StatusActive)
+	}
+	if StatusUserDeleted != "user_deleted" {
+		t.Errorf("StatusUserDeleted = %q, want \"user_deleted\"", StatusUserDeleted)
+	}
+}