Choose the expiry: --expires 7d|30d|90d

Meeting notes want a week, docs want longer. `htmldoc file.html --expires 90d`, within a cap, and the same option in the dashboard upload form.

`--update` would keep resetting to the page's own expiry length.

👍 if you want this. Say which length you would use most.

Start with the CLI entry point handling `htmldoc file.html --expires 90d`, then trace the dashboard upload form and the existing `--update` behavior. Confirm how expiry lengths and the cap are represented before changing both entry points. Done means the requested expiry choices work in the CLI and upload form, and updates preserve the page's selected expiry length.

Scritto dal modello di indicizzazione a partire dal testo della issue.