Add TTL cleanup for expired AI sessions on startup

[begelundmuller]

On runtime start, deletes AI sessions (and their messages) whose newest message is older than 90 days.

Contributes to https://linear.app/rilldata/issue/PLAT-412/handle-issues-with-large-ai-message-contents

Checklist:

• Covered by tests
• Ran it and it works as intended
• Reviewed the diff before requesting a review
• Checked for unhandled edge cases
• Linked the issues it closes
• Checked if the docs need to be updated. If so, create a separate Linear DOCS issue
• Intend to cherry-pick into the release branch
• I'm proud of this work!