Item expiry and pin-preservation

expired items are now automatically deleted on program startup.

an entry is considered expired when the current time is more than its expires_at value.
when pinned, the expires_at attribute is nulled, meaning this entry will never expire.

the “Clear all” shortcut Ctrl+Shift+Del now respects pinned entries. it clears everything but leaves pinned entries untouched.

Next

the ephemeral db shenanigans.
also, i have decided that Squirrel needs a settings panel in the frontend for easier config modification. that may or may not happen but it is tempting to do it.