feat(scanner): store EPUB reading direction at scan time

Fixed-layout EPUBs lean on the reading_direction column (the web reader
forces book.dir = rtl from it when the file didn't set direction
itself), but the scanner never populated it for EPUBs - only ComicInfo
fed it. Meanwhile real Japanese EPUBs declare page-progression-direction
on the OPF spine, which foliate reads client-side but nothing stored.

Read the spine attribute in the structured OPF parser and map it into
ReadingDirection in parseOPFContent (EPUB2/3, case-insensitive, plus
'right-to-left'/'left-to-right' spellings); undeclared stays empty
rather than forcing ltr, preserving the editor's Auto default. Sidecar
OPFs are metadata-only documents without spines, so the Calibre path is
a no-op. The merge gap-fill copies an embedded-only direction into a
blank sidecar field, and hand-set values keep winning through the
existing OverrideReadingDirection protection.

Tests: declared rtl/RTL/ltr, undeclared and unknown values staying
empty, plus sidecar-wins vs embedded-fills merge cases. Existing manga
EPUBs declaring rtl (verified live in-library) pick the value up on
their next scan, feeding the API and reader config mobile clients
consume.

internal/services/media_scanner.go

@@ -1209,6 +1209,9 @@ func (s *MediaScanner) mergeMetadata(path string, calibreMetadata *MediaMetadata
 			if metadata.SeriesNumber == 0 && embedded.SeriesNumber != 0 {
 				metadata.SeriesNumber = embedded.SeriesNumber
 			}
+			if metadata.ReadingDirection == "" && embedded.ReadingDirection != "" {
+				metadata.ReadingDirection = embedded.ReadingDirection
+			}
 			if metadata.PublishDate.IsZero() && !embedded.PublishDate.IsZero() {
 				metadata.PublishDate = embedded.PublishDate
 			}
@@ -1866,6 +1869,12 @@ func parseOPFContent(content []byte) (*MediaMetadata, error) {
 	if title := opf.selectTitle(); title != "" {
 		metadata.Title = title
 	}
+	// Reading direction from the OPF spine's page-progression-direction.
+	// Sidecar OPFs are metadata-only documents without a spine, so this is a
+	// no-op for them and only fires on real books.
+	if dir := opf.pageProgressionDirection(); dir != "" {
+		metadata.ReadingDirection = dir
+	}
 	// Author (first creator)
 	if len(md.Creators) > 0 {
 		metadata.Author = md.Creators[0]