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/domain/bookmark.go

@@ -0,0 +1,16 @@
+package domain
+
+import "time"
+
+type Bookmark struct {
+	ID          uint      `gorm:"primarykey" json:"id"`
+	UserID      uint      `gorm:"not null;index" json:"userId"`
+	Title       string    `gorm:"not null" json:"title"`
+	URL         string    `gorm:"not null" json:"url"`
+	Description string    `json:"description"`
+	Icon        string    `json:"icon"`
+	Category    string    `gorm:"default:'默认'" json:"category"`
+	SortOrder   int       `gorm:"default:0" json:"sortOrder"`
+	CreatedAt   time.Time `json:"createdAt"`
+	UpdatedAt   time.Time `json:"updatedAt"`
+}