feat(enterprise): add trace processor RAM admission

[Gracker]

Summary

• add trace_processor RAM budget estimation and admission control for owned processor creation
• sample startup/current/peak RSS from child trace_processor processes and surface RAM budget stats
• persist observed processor RSS into frontend/manual leases and expose queue/holder/crash summaries from /api/traces/stats
• mark enterprise runtime isolation item 4.7 complete; 4.3 large-trace RSS benchmark remains blocked by missing required trace matrix

Verification

• cd backend && PATH="$HOME/.nvm/versions/node/v24.15.0/bin:$PATH" npx jest --runInBand src/services/__tests__/traceProcessorRamBudget.test.ts src/services/__tests__/workingTraceProcessor.enterpriseIsolation.test.ts src/routes/__tests__/enterpriseTraceMetadataRoutes.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 build
• git diff --check
• cd backend && PATH="$HOME/.nvm/versions/node/v24.15.0/bin:$PATH" npm run test:core
• 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