Introduce pre-computed daily usage rollups from sandbox_metrics_snapshots.
An hourly background worker aggregates completed days, while today's
usage is computed live from snapshots at query time for freshness.
Backend: new daily_usage table, rollup worker, UsageService, and
GET /v1/capsules/usage endpoint with date range filtering (up to 92 days).
Frontend: replace Usage page placeholder with bar charts (Chart.js),
summary total cards, and preset/custom date range controls.
Extracts Mailer interface, EmailData, and Button to pkg/email/types.go
so the cloud repo can use them via ServerContext. internal/email re-exports
the types as aliases so existing callers are unchanged. Also fixes
pre-existing lint errors (unchecked rollback and deadline calls).
