Database hardening
- Added concurrent event indexes:
- message_events_workspace_created_idx
- message_click_events_workspace_occurred_idx
- message_click_events_message_occurred_idx
- Confirmed existing rollup uniqueness for:
- analytics_daily_message_volume (workspace_id, date)
- analytics_daily_agent_usage (workspace_id, date, agent_id)
- analytics_daily_token_spend (workspace_id, date, model)