feat(opds): serve comic archives in native format with correct mime types

The OPDS device catalog previously hard-coded every acquisition link as
application/epub+zip and always offered kepub/pdf alternate links, which
is wrong for comic archives (cbz/cbr/cb7/cbt) and other non-epub media.

- Resolve the acquisition mime type from the media item's stored
  mime_type (falling back to format_mimetype, then epub) instead of
  assuming epub
- Only offer reflowable conversions (kepub for kobo, pdf) for ebooks;
  comic archives are served as-is in their native format
- Derive the native format label (epub/pdf/cbz/cbr/...) from the file
  path in ListFormats rather than always reporting epub
- Add resolveMimeType, isComicArchive, and formatLabelFromPath helpers

internal/handlers/opds.go

@@ -67,6 +67,55 @@ func appendToken(url, token string) string {
 	return url + "?token=" + token
 }
 
+// resolveMimeType returns the mime type for a media item, preferring the stored
+// mime_type, then format_mimetype, and finally falling back to EPUB.
+func resolveMimeType(mime, formatMime pgtype.Text) string {
+	if mime.Valid && mime.String != "" {
+		return mime.String
+	}
+	if formatMime.Valid && formatMime.String != "" {
+		return formatMime.String
+	}
+	return "application/epub+zip"
+}
+
+// isComicArchive reports whether a format group represents a comic/manga
+// archive (cbz/cbr/cb7/cbt). Comic archives are served in their native format
+// and should not be offered as EPUB/KEPUB/PDF conversions.
+func isComicArchive(formatGroup string) bool {
+	return strings.EqualFold(formatGroup, "comic_archive")
+}
+
+// formatLabelFromPath derives a short format label (e.g. "epub", "cbz") from a
+// file path's extension, defaulting to "epub" when it cannot be determined.
+func formatLabelFromPath(path string) string {
+	ext := strings.ToLower(filepath.Ext(path))
+	switch ext {
+	case ".epub":
+		return "epub"
+	case ".pdf":
+		return "pdf"
+	case ".cbz":
+		return "cbz"
+	case ".cbr":
+		return "cbr"
+	case ".cb7":
+		return "cb7"
+	case ".cbt":
+		return "cbt"
+	case ".mobi":
+		return "mobi"
+	case ".azw", ".azw3":
+		return "azw3"
+	case ".txt":
+		return "txt"
+	case "":
+		return "epub"
+	default:
+		return strings.TrimPrefix(ext, ".")
+	}
+}
+
 // GetDeviceCatalog returns the OPDS catalog feed for a device
 func (h *OPDSHandler) GetDeviceCatalog(c *echo.Context) error {
 	deviceID := c.Param("deviceId")
@@ -189,18 +238,22 @@ func (h *OPDSHandler) GetDeviceCatalog(c *echo.Context) error {
 			entry.SetSummary(item.Description.String)
 		}
 
-		// Add acquisition links
+		// Add acquisition link using the item's real mime type
 		downloadURL := appendToken(fmt.Sprintf("%s/devices/%s/download/%s", opdsBaseURL, deviceID, bookUUID), token)
-		entry.AddAcquisitionLink(downloadURL, "application/epub+zip")
+		entry.AddAcquisitionLink(downloadURL, resolveMimeType(item.MimeType, item.FormatMimetype))
 
-		if device.DeviceType == "kobo" {
-			kepubURL := downloadURL + "&format=kepub"
-			entry.AddAlternateLink(kepubURL, "application/vnd.kobo+xml+zip")
+		// Only offer reflowable conversions (kepub/pdf) for ebooks; comic
+		// archives are served as-is in their native format.
+		if !isComicArchive(item.FormatGroup) {
+			if device.DeviceType == "kobo" {
+				kepubURL := downloadURL + "&format=kepub"
+				entry.AddAlternateLink(kepubURL, "application/vnd.kobo+xml+zip")
+			}
+
+			pdfURL := downloadURL + "&format=pdf"
+			entry.AddAlternateLink(pdfURL, "application/pdf")
 		}
 
-		pdfURL := downloadURL + "&format=pdf"
-		entry.AddAlternateLink(pdfURL, "application/pdf")
-
 		// Add canonical identifier
 		entry.SetIdentifier(bookUUID)
 
@@ -318,9 +371,11 @@ func (h *OPDSHandler) SearchDeviceCatalog(c *echo.Context) error {
 		}
 
 		downloadURL := appendToken(fmt.Sprintf("%s/devices/%s/download/%s", opdsBaseURL, deviceID, bookUUID), token)
-		entry.AddAcquisitionLink(downloadURL, "application/epub+zip")
+		entry.AddAcquisitionLink(downloadURL, resolveMimeType(item.MimeType, item.FormatMimetype))
 
-		if device.DeviceType == "kobo" {
+		// Only offer kepub conversion for ebooks; comic archives are served
+		// as-is in their native format.
+		if !isComicArchive(item.FormatGroup) && device.DeviceType == "kobo" {
 			kepubURL := downloadURL + "&format=kepub"
 			entry.AddAlternateLink(kepubURL, "application/vnd.kobo+xml+zip")
 		}
@@ -705,14 +760,14 @@ func (h *OPDSHandler) ListFormats(c *echo.Context) error {
 
 	formatList := []FormatInfo{}
 
-	// Add EPUB format (always available if media item exists)
+	// Add the primary/native format (always available if media item exists)
 	fileSize := int64(0)
 	if mediaItem.FileSize.Valid {
 		fileSize = mediaItem.FileSize.Int64
 	}
 
 	formatList = append(formatList, FormatInfo{
-		FormatType: "epub",
+		FormatType: formatLabelFromPath(mediaItem.FilePath),
 		FilePath:   mediaItem.FilePath,
 		FileSha256: func() string {
 			if mediaItem.FileSha256.Valid {