feat(enterprise): add trace processor SQL worker

[Gracker]

Summary

• Add a per-trace-processor SQL worker backed by worker_threads in source/dev and compiled dist paths, with inline fallback only for tests/custom executors.
• Replace the old per-processor FIFO promise chain with a non-preemptive P0/P1/P2 priority queue: the running query is never interrupted, but queued P0 work dispatches before queued P1/P2 work.
• Route WorkingTraceProcessor, ExternalRpcProcessor, manual SQL, and backend lease proxy /api/tp/:leaseId/query through the worker queue.
• Add protobuf helpers for QueryArgs decode and QueryResult encode so raw proxy/test paths can preserve Perfetto HTTP RPC protobuf payloads.
• Mark README §0.4.6 complete. §0.4.3 remains blocked on real 100MB / 500MB / 1GB trace benchmark coverage.

Stacking

• Stacked on feat(enterprise): proxy trace processor leases #87 / feature/enterprise-multi-tenant-tp-proxy.
• This PR only covers §0.4.6.

Validation

• cd backend && npx jest --runInBand src/services/__tests__/traceProcessorSqlWorker.test.ts src/services/__tests__/traceProcessorProtobuf.test.ts src/services/__tests__/workingTraceProcessor.enterpriseIsolation.test.ts src/routes/__tests__/traceProcessorProxyRoutes.test.ts
• cd backend && npm run typecheck
• cd backend && npm run build
• source/dev worker_thread smoke with npx tsx
• compiled dist/ worker_thread smoke with node dist/...
• cd backend && npm run test:core
• cd backend && npm run test:scene-trace-regression
• npm run verify:pr
• git diff --check

[Gracker]

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