feat(frontend): display upstream model in usage table and distribution charts

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.

frontend/src/i18n/locales/en.ts

@@ -718,11 +718,14 @@ export default {
     exporting: 'Exporting...',
     preparingExport: 'Preparing export...',
     model: 'Model',
+    requestedModel: 'Requested',
+    upstreamModel: 'Upstream',
     reasoningEffort: 'Reasoning Effort',
     endpoint: 'Endpoint',
     endpointDistribution: 'Endpoint Distribution',
     inbound: 'Inbound',
     upstream: 'Upstream',
+    mapping: 'Mapping',
     path: 'Path',
     inboundEndpoint: 'Inbound Endpoint',
     upstreamEndpoint: 'Upstream Endpoint',