feat: 新增 SMTP/IMAP/POP3 协议调用日志（含攻击分析筛选）

- 每个连接记录一条日志：协议、端口、来源 IP、用户名、成功/失败、
  失败原因（密码错误/IP封禁/中继被拒/发件人伪造/未认证发信等）、
  操作摘要、消息数与会话时长
- 管理后台新增「协议日志」页：按协议/状态/IP/用户名/时间筛选，
  今日与历史成功/失败统计卡片，分页查看，可手动清理
- 后台每 6 小时自动清理超出 protocol_log_keep_days（默认30天）
  的日志；新增 [web] protocol_log_keep_days 配置项
- 修复 POP3 认证既有 bug：handleUSER 丢弃邮箱域名导致 PASS 永远失败
- 新增 store 单测、SMTP/POP3 端到端测试与模板渲染测试

internal/db/db.go

@@ -46,7 +46,7 @@ func InitDB(cfg config.DatabaseConfig, storageCfg config.StorageConfig) (*gorm.D
 	}
 
 	// Auto-migrate all models
-	if err := db.AutoMigrate(&User{}, &Domain{}, &Message{}, &Attachment{}, &BanEntry{}, &OutboundMessage{}); err != nil {
+	if err := db.AutoMigrate(&User{}, &Domain{}, &Message{}, &Attachment{}, &BanEntry{}, &OutboundMessage{}, &ProtocolLog{}); err != nil {
 		return nil, fmt.Errorf("数据库迁移失败: %w", err)
 	}
 

internal/db/models.go

@@ -122,6 +122,34 @@ type BanEntry struct {
 // TableName specifies the table name for BanEntry.
 func (BanEntry) TableName() string { return "ban_entries" }
 
+// Protocol log statuses.
+const (
+	ProtocolSMTP = "smtp"
+	ProtocolIMAP = "imap"
+	ProtocolPOP3 = "pop3"
+)
+
+// ProtocolLog records one SMTP/IMAP/POP3 connection session: auth result,
+// failure reason and source IP, for admin analysis of attacks/abuse.
+type ProtocolLog struct {
+	ID         uint      `gorm:"primaryKey" json:"id"`
+	Protocol   string    `gorm:"size:16;index;not null" json:"protocol"` // smtp | imap | pop3
+	Port       int       `json:"port"`                                   // 25/465/587/143/993/110/995
+	ClientIP   string    `gorm:"size:64;index;not null" json:"client_ip"`
+	Username   string    `gorm:"size:255;index" json:"username"`
+	Success    bool      `gorm:"index" json:"success"`
+	FailReason string    `gorm:"size:512" json:"fail_reason"`
+	Detail     string    `gorm:"size:2048" json:"detail"`
+	MsgCount   int       `json:"msg_count"`
+	DurationMs int64     `json:"duration_ms"`
+	CreatedAt  time.Time `gorm:"index" json:"created_at"`
+}
+
+// TableName specifies the table name for ProtocolLog.
+func (ProtocolLog) TableName() string {
+	return "protocol_logs"
+}
+
 // Attachment represents a file attached to an email message.
 type Attachment struct {
 	ID          uint      `gorm:"primaryKey" json:"id"`