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/services/worker.go

@@ -221,7 +221,8 @@ func (w *Worker) processJob(job *Job) {
 	}
 	w.mu.Unlock()
 
-	job.StartedAt = new(time.Now())
+	started := time.Now()
+	job.StartedAt = &started
 
 	w.mu.Lock()
 	if result, exists := w.results[job.ID]; exists {
@@ -253,7 +254,8 @@ func (w *Worker) processJob(job *Job) {
 		err = fmt.Errorf("unknown job type: %s", job.Type)
 	}
 
-	job.CompletedAt = new(time.Now())
+	completed := time.Now()
+	job.CompletedAt = &completed
 	job.Error = err
 	job.Result = result