backend: add bookmark CRUD with public list endpoint

New bookmarks feature: domain model, repository, service and handler
supporting list/create/update/delete. Public endpoint exposes the
admin user's bookmarks for the homepage navigation grid; authenticated
endpoints scope by user. Dev Dockerfile drops air for plain `go run`
and uses goproxy.cn to avoid build failures on the deploy host.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

backend/internal/db/db.go

@@ -18,7 +18,7 @@ func Init(cfg *config.Config) *gorm.DB {
 		log.Fatalf("failed to connect database: %v", err)
 	}
 
-	if err := DB.AutoMigrate(&domain.User{}); err != nil {
+	if err := DB.AutoMigrate(&domain.User{}, &domain.Bookmark{}); err != nil {
 		log.Fatalf("failed to migrate database: %v", err)
 	}