feat(enterprise): expose per-lease queue stats

[Gracker]

Summary

• report queue length per TraceProcessorLease instead of using only the shared trace queue
• keep shared frontend queue stats separate from isolated report-generation lease queue stats
• add enterprise stats regression coverage for frontend shared P0 + report_generation isolated P2 queues

Validation

• cd backend && PATH="$HOME/.nvm/versions/node/v24.15.0/bin:$PATH" npx jest --runInBand --forceExit src/routes/__tests__/enterpriseTraceMetadataRoutes.test.ts src/services/__tests__/traceProcessorLeaseModeDecision.test.ts src/services/__tests__/traceProcessorSqlWorker.test.ts
• cd backend && PATH="$HOME/.nvm/versions/node/v24.15.0/bin:$PATH" npm run typecheck
• cd backend && PATH="$HOME/.nvm/versions/node/v24.15.0/bin:$PATH" npm run test:core
• PATH="$HOME/.nvm/versions/node/v24.15.0/bin:$PATH" npm run verify:pr

TODO

• Advances README §0.4.12 / §11.11: report-generation isolated work is now observable separately from frontend shared queue state.
• README §0.4.12 remains unchecked until every §11.11 row is verified.
• README §0.4.3 remains blocked on the required large trace RSS matrix.

[Gracker]

已通过 commit 67856ec (merge) + 023a84a (frontend rebuild) 合入 main，关闭此 PR。