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/repository/audit_log_repository.go

@@ -0,0 +1,56 @@
+package repository
+
+import (
+	"context"
+
+	"backupx/server/internal/model"
+	"gorm.io/gorm"
+)
+
+type AuditLogListOptions struct {
+	Category string
+	Limit    int
+	Offset   int
+}
+
+type AuditLogListResult struct {
+	Items []model.AuditLog `json:"items"`
+	Total int64            `json:"total"`
+}
+
+type AuditLogRepository interface {
+	Create(ctx context.Context, log *model.AuditLog) error
+	List(ctx context.Context, opts AuditLogListOptions) (*AuditLogListResult, error)
+}
+
+type gormAuditLogRepository struct {
+	db *gorm.DB
+}
+
+func NewAuditLogRepository(db *gorm.DB) AuditLogRepository {
+	return &gormAuditLogRepository{db: db}
+}
+
+func (r *gormAuditLogRepository) Create(_ context.Context, log *model.AuditLog) error {
+	return r.db.Create(log).Error
+}
+
+func (r *gormAuditLogRepository) List(_ context.Context, opts AuditLogListOptions) (*AuditLogListResult, error) {
+	query := r.db.Model(&model.AuditLog{})
+	if opts.Category != "" {
+		query = query.Where("category = ?", opts.Category)
+	}
+	var total int64
+	if err := query.Count(&total).Error; err != nil {
+		return nil, err
+	}
+	limit := opts.Limit
+	if limit <= 0 {
+		limit = 50
+	}
+	var items []model.AuditLog
+	if err := query.Order("created_at DESC").Offset(opts.Offset).Limit(limit).Find(&items).Error; err != nil {
+		return nil, err
+	}
+	return &AuditLogListResult{Items: items, Total: total}, nil
+}