feat: progressive drill-down dashboard with RED metrics and SLA compliance (#94)
Three-level dashboard driven by sidebar selection:
- L1 (no selection): all-apps overview with health table, per-app charts
- L2 (app selected): route performance table, error velocity, top errors
- L3 (route selected): processor table, latency heatmap data, bottleneck KPI
Backend: 3 new endpoints (timeseries/by-app, timeseries/by-route, errors/top),
per-app SLA settings (app_settings table, V12 migration), exact SLA compliance
from executions hypertable, error velocity with acceleration detection.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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