feat: Go 重写后端，替换 Python FastAPI

用 Go (Gin + GORM + SQLite) 重写整个后端：
- 单二进制部署，不依赖 Python/pip/SDK
- net/http 原生客户端，无 Cloudflare TLS 指纹问题
- 多阶段 Dockerfile：Node 构建前端 + Go 构建后端 + Alpine 运行
- 内存占用从 ~95MB 降至 ~3MB
- 完整保留所有 API 路由、JWT 认证、API Key 权限、审计日志

models/system_setting.go

@@ -0,0 +1,37 @@
+package models
+
+import (
+	"time"
+
+	"gorm.io/gorm"
+	"gorm.io/gorm/clause"
+)
+
+type SystemSetting struct {
+	ID        uint      `json:"id" gorm:"primaryKey;autoIncrement"`
+	Key       string    `json:"key" gorm:"column:key;type:varchar(255);uniqueIndex:ix_system_settings_key;not null"`
+	Value     string    `json:"value" gorm:"type:text;not null"`
+	Encrypted bool      `json:"encrypted" gorm:"not null"`
+	UpdatedAt time.Time `json:"updated_at" gorm:"not null;autoUpdateTime"`
+}
+
+func GetSetting(db *gorm.DB, key string) string {
+	var setting SystemSetting
+	result := db.Where("`key` = ?", key).First(&setting)
+	if result.Error != nil {
+		return ""
+	}
+	return setting.Value
+}
+
+func SetSetting(db *gorm.DB, key, value string, encrypted bool) {
+	setting := SystemSetting{
+		Key:       key,
+		Value:     value,
+		Encrypted: encrypted,
+	}
+	db.Clauses(clause.OnConflict{
+		Columns:   []clause.Column{{Name: "key"}},
+		DoUpdates: clause.AssignmentColumns([]string{"value", "encrypted", "updated_at"}),
+	}).Create(&setting)
+}