fix: replace invalid new(expression) calls with proper pointer allocation

Go's new() builtin takes a type and allocates a zero value — it cannot
wrap an expression. All instances of new(someExpression) were compile
errors. Replace each with a local variable assignment and address-of
operator.

Affected files:
- handlers/koreader.go: progress field pointers (Chapter, Page, etc.)
- handlers/kobo.go: pagesRemaining pointer
- handlers/queue.go: uuidPtrToString and timestamptzPtrToString helpers
- router/reader.go: bookmark pageNumber and chapterNumber pointers
- services/media_scanner.go: validation error message pointers
- services/worker.go: StartedAt and CompletedAt timestamps
- sync/offline.go: GetDeviceStatus return pointer
- tests/device_test.go: SyncEnabled and SyncFrequencyMinutes pointers

internal/handlers/koreader.go

@@ -602,19 +602,24 @@ func (h *KOReaderHandler) GetMetadata(c *echo.Context) error {
 		progressData.Epubcfi = &progress.Epubcfi.String
 	}
 	if progress.Chapter.Valid {
-		progressData.Chapter = new(int(progress.Chapter.Int32))
+		progress := int(progress.Chapter.Int32)
+		progressData.Chapter = &progress
 	}
 	if progress.ChapterProgress.Valid {
-		progressData.ChapterProgress = new(progress.ChapterProgress.Float64)
+		progress := progress.ChapterProgress.Float64
+		progressData.ChapterProgress = &progress
 	}
 	if progress.CharacterOffset.Valid {
-		progressData.Character = new(progress.CharacterOffset.Int64)
+		progress := progress.CharacterOffset.Int64
+		progressData.Character = &progress
 	}
 	if progress.CurrentPage.Valid {
-		progressData.Page = new(int(progress.CurrentPage.Int32))
+		progress := int(progress.CurrentPage.Int32)
+		progressData.Page = &progress
 	}
 	if progress.TotalPages.Valid {
-		progressData.TotalPages = new(int(progress.TotalPages.Int32))
+		progress := int(progress.TotalPages.Int32)
+		progressData.TotalPages = &progress
 	}
 
 	annotations, err := h.db.GetAnnotationsForBook(c.Request().Context(), database.GetAnnotationsForBookParams{
@@ -694,7 +699,8 @@ func (h *KOReaderHandler) GetLibrary(c *echo.Context) error {
 		if err == nil {
 			percentRead = progress.Percentage.Float64
 			if progress.TotalPages.Valid && progress.CurrentPage.Valid {
-				pagesRemaining = new(int(progress.TotalPages.Int32 - progress.CurrentPage.Int32))
+				pages := int(progress.TotalPages.Int32 - progress.CurrentPage.Int32)
+				pagesRemaining = &pages
 			}
 			if progress.LastReadAt.Valid {
 				lastModified = progress.LastReadAt.Time.Format(time.RFC3339)