refactor(handlers): replace temporary variable pointer pattern with new() builtin

Simplify pointer creation across kobo, koreader, and queue handlers by
replacing the two-step pattern (assign to local, then take address) with
inline new() calls. This reduces verbosity without changing behavior:

Before:
  remaining := int(a - b)
  pagesRemaining = &remaining

After:
  pagesRemaining = new(int(a - b))

Covers page calculations, chapter/progress fields, UUID formatting,
and timestamp string conversions.

internal/handlers/queue.go

@@ -310,8 +310,7 @@ func uuidPtrToString(u pgtype.UUID) *string {
 	if !u.Valid {
 		return nil
 	}
-	s := uuid.UUID(u.Bytes).String()
-	return &s
+	return new(uuid.UUID(u.Bytes).String())
 }
 
 func textPtrToString(t pgtype.Text) *string {
@@ -325,6 +324,5 @@ func timestamptzPtrToString(t pgtype.Timestamptz) *string {
 	if !t.Valid {
 		return nil
 	}
-	s := t.Time.Format("2006-01-02T15:04:05Z07:00")
-	return &s
+	return new(t.Time.Format("2006-01-02T15:04:05Z07:00"))
 }