feat(backend): add EPUB CFI and percentage to reading progress

Enhance reading progress tracking to support EPUB-specific location data:
- Add epubcfi field to store EPUB Canonical Fragment Identifier
- Add percentage field for normalized position across formats
- Update UpdateReadingProgress API handler to accept new fields
- Modify database queries to persist additional progress metadata

This enables precise position tracking in reflowable EPUB content
where page numbers are insufficient for accurate bookmarking.

internal/database/queries.sql.go

@@ -9579,6 +9579,8 @@ type UpdateReadingProgressParams struct {
 	UserID      pgtype.UUID `db:"user_id" json:"user_id"`
 	CurrentPage pgtype.Int4 `db:"current_page" json:"current_page"`
 	TotalPages  pgtype.Int4 `db:"total_pages" json:"total_pages"`
+	Epubcfi     string      `json:"epubcfi"`
+	Percentage  float64     `json:"percentage"`
 }
 
 func (q *Queries) UpdateReadingProgress(ctx context.Context, arg UpdateReadingProgressParams) (ReadingProgress, error) {