- Add get_impact_summary/1: aggregates current outage subscriber/MRR impact
- Add list_active_incidents/1: down devices enriched with Gaiia/Preseem data, sorted by MRR
- Add get_site_impact_summaries/1: per-site health with QoE, affected subs, MRR at risk
- Dashboard top stats: MRR at Risk and Subscribers Affected cards
- Active Incidents section: color-coded by severity, shows duration/subs/MRR/QoE
- Site Health Grid: green/yellow/red health dots based on uptime and QoE scores
