Add download counter for datasets

[drorganvidez]

Description:
Allow tracking how many times a dataset has been downloaded by incrementing a counter each time a download is requested.
This gives authors and users a simple way to measure dataset popularity.

Details:

• Add a new field download_count (default = 0) to the dataset model.
• Each time a dataset file (or ZIP) is downloaded, increment download_count in the database.
• Expose this value in the dataset API and show it in the dataset detail page.
• Optionally, add a simple endpoint /datasets/{id}/stats that returns downloads, views, etc.