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_opf.go

@@ -58,7 +58,8 @@ type opfDocument struct {
 		Items []opfItem `xml:"item"`
 	} `xml:"manifest"`
 	Spine struct {
-		Itemrefs []struct {
+		PageProgressionDirection string `xml:"page-progression-direction,attr"`
+		Itemrefs                 []struct {
 			IDRef string `xml:"idref,attr"`
 		} `xml:"itemref"`
 	} `xml:"spine"`
@@ -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))