feat(canvas): 绘图工具支持自定义 tldraw License Key (#100)

* feat(canvas): 绘图工具 - 支持自定义 tldraw License Key

* docs(canvas): 绘图工具 - 添加 License Key 页面截图

backend/internal/service/drawing_engine_test.go

@@ -1,6 +1,16 @@
 package service
 
-import "testing"
+import (
+	"encoding/json"
+	"strings"
+	"testing"
+
+	"infinite-canvas/backend/internal/model"
+	"infinite-canvas/backend/internal/repository"
+
+	"gorm.io/driver/sqlite"
+	"gorm.io/gorm"
+)
 
 func TestDefaultDrawingEngineUsesExcalidraw(t *testing.T) {
 	setting := defaultDrawingEngineSetting()
@@ -19,3 +29,53 @@ func TestValidateDrawingEngineSetting(t *testing.T) {
 		t.Fatal("validate unknown engine = nil")
 	}
 }
+
+func TestUpdateDrawingEngineSettingKeepsDefaultEngineAndStoresTldrawLicenseSeparately(t *testing.T) {
+	db, err := gorm.Open(sqlite.Open(":memory:"), &gorm.Config{})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if sqlDB, err := db.DB(); err == nil {
+		sqlDB.SetMaxOpenConns(1)
+	}
+	if err := db.AutoMigrate(&model.SystemSetting{}, &model.AdminAuditEvent{}); err != nil {
+		t.Fatal(err)
+	}
+	repo := repository.New(db)
+	svc := New(repo, t.TempDir())
+	actor := &model.User{ID: "admin-1", Role: model.UserRoleAdmin}
+
+	setting, err := svc.UpdateDrawingEngineSetting(actor, DrawingEngineSetting{
+		DefaultEngine:    DrawingEngineExcalidraw,
+		TldrawLicenseKey: "license-key-for-test",
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if setting.DefaultEngine != DrawingEngineExcalidraw {
+		t.Fatalf("default engine = %q, want %q", setting.DefaultEngine, DrawingEngineExcalidraw)
+	}
+	if setting.TldrawLicenseKey != "license-key-for-test" {
+		t.Fatalf("license key = %q", setting.TldrawLicenseKey)
+	}
+
+	licenseSetting, err := repo.SystemSetting(tldrawLicenseKeySettingKey)
+	if err != nil {
+		t.Fatal(err)
+	}
+	var savedLicenseKey string
+	if err := json.Unmarshal([]byte(licenseSetting.ValueJSON), &savedLicenseKey); err != nil {
+		t.Fatal(err)
+	}
+	if savedLicenseKey != "license-key-for-test" {
+		t.Fatalf("stored license key = %q", savedLicenseKey)
+	}
+
+	engineSetting, err := repo.SystemSetting(drawingEngineSettingKey)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if strings.Contains(engineSetting.ValueJSON, "license-key-for-test") {
+		t.Fatalf("drawing engine setting must not contain the license key: %s", engineSetting.ValueJSON)
+	}
+}