Follow-up to #26543. The sessions table does not have an updated_at
column (see hermes_state.py — only started_at/ended_at), so
row.get('updated_at') always returned None and the str() coercion was
dead code. Use datetime.now(UTC).isoformat() instead, which reflects
exactly what the field means here: 'the title was refreshed at this
moment'. Drop the dead coercion.
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