Auto-end stale sessions

Our session data is polluted with unrealistic entries. Some sessions show durations of 490+ hours because users close/kill the app mid-session and the end_time never gets set. Out of 51 sessions in the last 30 days, the average duration is ~11 hours — clearly skewed by abandoned sessions.

This corrupts stats, makes analytics unreliable, and gives returning users a confusing experience: when they reopen the app after days, the old session resumes as if nothing happened.