feat(dashboard): pool capacity section with per-account detail and observed routing order (PR #432)

Replaces the overview's account-wide pool cards with an expandable
"Pool capacity" section covering both account-wide 5h/7d pools and
per-model-family weekly-scoped pools (discovered from the data).
Expanded rows show per-account segment bars, reset countdowns, and
pool averages. Adds a proxy-side observed routing order recorder
(process-local, display-only) served via GET /api/routing/observations
and rendered as a table of each family's actual serving order.