Preview image doesn't update when the underlying PDF changes outside of PDFKeeper

[rffrasca]

Describe the bug
When a document’s PDF file is modified outside of PDFKeeper—such as during a database flashback operation—the preview image on the main form does not update. PDFKeeper continues displaying the old preview image.

Observed Behavior

• The old PDF remains in the FileCache on disk.
• The old PDF also remains in the in‑memory FileCache collection.
• As a result, the preview image shown in the main form does not reflect the updated PDF stored in the database.

To Reproduce
Steps to reproduce the behavior:

1. Open a document in PDFKeeper so that its preview is generated.
2. Perform an external operation that changes the PDF (e.g., database flashback).
3. Return to PDFKeeper and select the same document.
4. The preview still shows the old PDF.

Expected Behavior
PDFKeeper should detect that the underlying PDF has changed and refresh:

• The cached PDF file on disk
• The in‑memory cached entry
• The preview image displayed on the main form

Database (please complete the following information):

• DBMS: Oracle, MySQL, SQL Server