fix(session): use datetime for cutoff in suspend_recently_active()

[zicochaos]

Summary

suspend_recently_active() compares entry.updated_at (a datetime object) with time.time() (a float), which raises TypeError: '>=' not supported between instances of 'datetime.datetime' and 'float' on every gateway startup.

Fix: use _now() - timedelta(seconds=max_age_seconds) instead of time.time() - max_age_seconds, matching the type of updated_at and the pattern used elsewhere in session.py (e.g. list_sessions()).

Test plan

• Start gateway — no more TypeError on startup
• Verified _now() and timedelta are already imported in session.py
• Pattern matches existing code at line 939 (list_sessions)

[teknium1]

Thanks for the contribution, @zicochaos!

Closing this as a duplicate of #8026 (by @tomqiaozc), which targets the same fix/feature. We're consolidating on that PR for review.

If you want to help push it over the line, please jump in there — or if you think your approach is better for a specific reason that isn't covered in the other PR, let us know and we can reopen.