Show upstream model mapping (requested -> upstream) in UsageTable with arrow notation. Add requested/upstream/mapping source toggle to ModelDistributionChart with lazy loading — only fetches data when user switches tab, with per-source cache invalidation on filter changes. Include upstream_model column in Excel export and i18n for en/zh.
Click on a group name, model name, or endpoint name in the distribution
tables to expand and show per-user usage breakdown (requests, tokens,
actual cost, standard cost).
Backend: new GET /admin/dashboard/user-breakdown API with group_id,
model, endpoint, endpoint_type filters.
Frontend: clickable rows with expand/collapse sub-table in all three
distribution charts.
