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]

internal/services/media_scanner_opf.go

@@ -58,6 +58,7 @@ type opfDocument struct {
 		Items []opfItem `xml:"item"`
 	} `xml:"manifest"`
 	Spine struct {
+		PageProgressionDirection string `xml:"page-progression-direction,attr"`
 		Itemrefs                 []struct {
 			IDRef string `xml:"idref,attr"`
 		} `xml:"itemref"`
@@ -184,6 +185,22 @@ func (d *opfDocument) readSeries() (series string, index float64) {
 	return series, index
 }
 
+// pageProgressionDirection returns the OPF spine's reading direction as
+// "rtl" or "ltr", or "" when the file declares none (callers treat that as
+// unknown, not as left-to-right). EPUB2/3 declare this on <spine>; it is
+// what foliate reads client-side, and the DB column feeds clients (and the
+// web reader's fixed-layout override) that need it up front.
+func (d *opfDocument) pageProgressionDirection() string {
+	switch strings.ToLower(strings.TrimSpace(d.Spine.PageProgressionDirection)) {
+	case "rtl", "right-to-left":
+		return "rtl"
+	case "ltr", "left-to-right", "default":
+		return "ltr"
+	default:
+		return ""
+	}
+}
+
 // itemByID returns manifest items with id, href and media-type, keyed by id.
 func (d *opfDocument) itemByID() map[string]opfItem {
 	m := make(map[string]opfItem, len(d.Manifest.Items))

internal/services/media_scanner_opf_test.go

@@ -221,3 +221,77 @@ func TestResolveOPFPath(t *testing.T) {
 		}
 	}
 }
+
+// TestParseOPFContentPageProgressionDirection verifies the EPUB spine's
+// page-progression-direction feeds ReadingDirection, and that an undeclared
+// direction stays empty rather than forcing left-to-right.
+func TestParseOPFContentPageProgressionDirection(t *testing.T) {
+	makeOPF := func(spineAttrs string) string {
+		return `<?xml version="1.0"?>
+<package xmlns="http://www.idpf.org/2007/opf" version="3.0">
+  <metadata xmlns:dc="http://purl.org/dc/elements/1.1/"><dc:title>Dir Test</dc:title></metadata>
+  <manifest><item id="p1" href="p1.xhtml" media-type="application/xhtml+xml"/></manifest>
+  <spine` + spineAttrs + `><itemref idref="p1"/></spine>
+</package>`
+	}
+	tests := []struct {
+		name       string
+		spineAttrs string
+		want       string
+	}{
+		{"rtl declared lowercase", ` page-progression-direction="rtl"`, "rtl"},
+		{"rtl declared uppercase", ` page-progression-direction="RTL"`, "rtl"},
+		{"ltr declared", ` page-progression-direction="ltr"`, "ltr"},
+		{"undeclared stays empty", ``, ""},
+		{"unknown value stays empty", ` page-progression-direction="sideways"`, ""},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			metadata, err := parseOPFContent([]byte(makeOPF(tt.spineAttrs)))
+			if err != nil {
+				t.Fatalf("parseOPFContent() error: %v", err)
+			}
+			if metadata.ReadingDirection != tt.want {
+				t.Errorf("ReadingDirection = %q, want %q", metadata.ReadingDirection, tt.want)
+			}
+		})
+	}
+}
+
+// TestMergeMetadataReadingDirectionGapFill verifies the sidecar wins when it
+// declares a direction, while an embedded-only direction fills the blank.
+func TestMergeMetadataReadingDirectionGapFill(t *testing.T) {
+	dir := t.TempDir()
+	epubPath := filepath.Join(dir, "dir.epub")
+	files := map[string]string{
+		"META-INF/container.xml": containerXML,
+		"OEBPS/package.opf": `<?xml version="1.0"?>
+<package xmlns="http://www.idpf.org/2007/opf" version="3.0">
+  <metadata xmlns:dc="http://purl.org/dc/elements/1.1/"><dc:title>RTL Book</dc:title></metadata>
+  <manifest><item id="p1" href="p1.xhtml" media-type="application/xhtml+xml"/></manifest>
+  <spine page-progression-direction="rtl"><itemref idref="p1"/></spine>
+</package>`,
+		"OEBPS/p1.xhtml": `<html><body><p>hi</p></body></html>`,
+	}
+	writeEPUB(t, epubPath, files)
+
+	s := NewMediaScanner(nil)
+
+	// Sidecar blank -> embedded rtl fills it
+	merged, err := s.mergeMetadata(epubPath, &MediaMetadata{Title: "Sidecar"})
+	if err != nil {
+		t.Fatalf("mergeMetadata() error: %v", err)
+	}
+	if merged.ReadingDirection != "rtl" {
+		t.Errorf("ReadingDirection = %q, want embedded rtl fill", merged.ReadingDirection)
+	}
+
+	// Sidecar ltr wins over embedded rtl
+	merged, err = s.mergeMetadata(epubPath, &MediaMetadata{Title: "Sidecar", ReadingDirection: "ltr"})
+	if err != nil {
+		t.Fatalf("mergeMetadata() error: %v", err)
+	}
+	if merged.ReadingDirection != "ltr" {
+		t.Errorf("ReadingDirection = %q, want sidecar ltr preserved", merged.ReadingDirection)
+	}
+}