Track last time a session was open in the session_store

### Describe the feature or problem you'd like to solve

Computer restarts and I need to restore my sessions. Love to review the sessions that were open to recover them.

### Proposed solution

Add a last_open column of type time to session store. Every 5 minutes, update the time in the column. Then I can see when each session was open regardless of whether I took any action in it.

### Example prompts or workflows

show me all the sessions that were open today
show me all the sessions that were open last week
look at all the open sessions from today - is any of them related to feature X?

### Additional context

_No response_

The issue names the session_store but no file or test paths. Start by locating that store and the session lifecycle entry point, then inspect how sessions are persisted and queried after a restart. Done means recording the last-open time periodically and supporting the stated today and last-week session workflows with tests.

Written by the indexing model from the issue text.