feat(enterprise): enforce quota and retention policies

[Gracker]

Summary

• enforce workspace quota_policy for trace uploads and analysis run admission
• add quota_exceeded terminal state for monthly budget/run exhaustion and max_budget_usd runtime termination
• apply retention_policy expiry to trace assets and report artifacts on enterprise read/write paths
• mark enterprise multi-tenant README §0.5.3 complete with implementation notes

Verification

• cd backend && PATH="$HOME/.nvm/versions/node/v24.15.0/bin:$PATH" npx jest --runInBand --forceExit src/services/tests/enterpriseQuotaPolicyService.test.ts src/routes/tests/enterpriseTraceMetadataRoutes.test.ts src/routes/tests/enterpriseReportRoutes.test.ts src/services/tests/analysisRunStore.test.ts src/routes/tests/memoryRoutes.test.ts src/services/providerManager/tests/providerRoutes.test.ts src/services/tests/enterpriseSchema.test.ts src/routes/tests/enterpriseApiKeyRoutes.test.ts src/routes/tests/enterpriseAuthRoutes.test.ts src/services/providerManager/tests/enterpriseProviderStore.test.ts
• cd backend && PATH="$HOME/.nvm/versions/node/v24.15.0/bin:$PATH" npm run typecheck
• git diff --check
• cd backend && PATH="$HOME/.nvm/versions/node/v24.15.0/bin:$PATH" npm run test:scene-trace-regression
• PATH="$HOME/.nvm/versions/node/v24.15.0/bin:$PATH" npm run verify:pr