fix(handlers): update OPDS download to use path resolution service

Update the DownloadBook function to properly resolve media file paths using
the LibraryService.ResolveMediaPath method instead of directly accessing the
FilePath field. This ensures correct file resolution after the migration to
relative path storage.

The change affects three code paths in the download handler:
- KEPUB conversion path
- Direct file serve path (non-EPUB with conversion service)
- Default EPUB path

Error handling added to return 404 when path resolution fails, preventing
potential errors when accessing non-existent files.

This fixes potential file access issues after the relative path storage
implementation.

internal/handlers/opds.go

@@ -402,7 +402,11 @@ func (h *OPDSHandler) DownloadBook(c echo.Context) error {
 			}
 		} else {
 			if h.conversionService != nil {
-				converted, err := h.conversionService.ConvertEPUBToKEPUB(c.Request().Context(), pgtype.UUID{Bytes: bookUUID, Valid: true}, mediaItem.FilePath)
+				resolvedPath, err := h.libraryService.ResolveMediaPath(c.Request().Context(), mediaItem.LibraryID, mediaItem.FilePath)
+				if err != nil {
+					return c.JSON(http.StatusNotFound, map[string]string{"error": "file not found"})
+				}
+				converted, err := h.conversionService.ConvertEPUBToKEPUB(c.Request().Context(), pgtype.UUID{Bytes: bookUUID, Valid: true}, resolvedPath)
 				if err != nil {
 					return c.JSON(http.StatusInternalServerError, map[string]string{"error": fmt.Sprintf("KEPUB conversion failed: %v", err)})
 				}
@@ -410,7 +414,10 @@ func (h *OPDSHandler) DownloadBook(c echo.Context) error {
 				fileSha256 = converted.SHA256
 				mimeType = "application/vnd.kobo+xml+zip"
 			} else {
-				filePath = mediaItem.FilePath
+				filePath, err = h.libraryService.ResolveMediaPath(c.Request().Context(), mediaItem.LibraryID, mediaItem.FilePath)
+				if err != nil {
+					return c.JSON(http.StatusNotFound, map[string]string{"error": "file not found"})
+				}
 				if mediaItem.MimeType.Valid {
 					mimeType = mediaItem.MimeType.String
 				}
@@ -435,7 +442,11 @@ func (h *OPDSHandler) DownloadBook(c echo.Context) error {
 		}
 	} else {
 		// Default: EPUB
-		filePath = mediaItem.FilePath
+		var err error
+		filePath, err = h.libraryService.ResolveMediaPath(c.Request().Context(), mediaItem.LibraryID, mediaItem.FilePath)
+		if err != nil {
+			return c.JSON(http.StatusNotFound, map[string]string{"error": "file not found"})
+		}
 		if mediaItem.MimeType.Valid {
 			mimeType = mediaItem.MimeType.String
 		}