feat: add community enhancements — password reset, audit logs, multi-source backup

Three community-requested features:

1. CLI password reset: `backupx reset-password --username admin --password xxx`
   Docker users can run via `docker exec`. No full app init needed.

2. Audit logging: async fire-and-forget audit trail for all key operations
   (login, CRUD on tasks/targets/records, settings changes).
   New UI page at /audit with category filter and pagination.

3. Multi-source path backup: file backup tasks now support multiple source
   directories packed into a single tar archive. Backward compatible with
   existing single sourcePath field.

server/internal/model/audit_log.go

@@ -0,0 +1,21 @@
+package model
+
+import "time"
+
+type AuditLog struct {
+	ID         uint      `gorm:"primaryKey" json:"id"`
+	UserID     uint      `gorm:"column:user_id;index" json:"userId"`
+	Username   string    `gorm:"column:username;size:64;not null" json:"username"`
+	Category   string    `gorm:"column:category;size:32;index;not null" json:"category"`
+	Action     string    `gorm:"column:action;size:64;not null" json:"action"`
+	TargetType string    `gorm:"column:target_type;size:32" json:"targetType"`
+	TargetID   string    `gorm:"column:target_id;size:64" json:"targetId"`
+	TargetName string    `gorm:"column:target_name;size:128" json:"targetName"`
+	Detail     string    `gorm:"column:detail;type:text" json:"detail"`
+	ClientIP   string    `gorm:"column:client_ip;size:45" json:"clientIp"`
+	CreatedAt  time.Time `gorm:"index" json:"createdAt"`
+}
+
+func (AuditLog) TableName() string {
+	return "audit_logs"
+}