feat(enterprise): persist trace metadata in db

[Gracker]

Summary

• close §0.3.3 by making enterprise mode persist trace metadata into trace_assets
• move enterprise trace files into data/{tenantId}/{workspaceId}/traces/ while preserving the legacy uploads/traces JSON path when enterprise mode is off
• add scoped DB-backed trace list/read/delete behavior and keep owner guard filtering on tenant/workspace/user context
• pass the actual stored trace file path into TraceProcessorService so processors load traces from the scoped data directory
• add enterprise upload/delete route coverage for DB metadata, scoped file placement, cross-workspace 404 behavior, and DB cleanup
• mark README §0.3.3 complete

Validation

• PATH="$HOME/.nvm/versions/node/v24.15.0/bin:$PATH" npx jest --runInBand --forceExit src/routes/__tests__/enterpriseTraceMetadataRoutes.test.ts in backend
• PATH="$HOME/.nvm/versions/node/v24.15.0/bin:$PATH" npx jest --runInBand --forceExit src/routes/__tests__/ownerGuardRoutes.test.ts src/routes/__tests__/workspaceResourceRoutes.test.ts src/routes/__tests__/enterpriseTraceMetadataRoutes.test.ts in backend
• PATH="$HOME/.nvm/versions/node/v24.15.0/bin:$PATH" npm run typecheck in backend
• PATH="$HOME/.nvm/versions/node/v24.15.0/bin:$PATH" npm run test:core in backend
• git diff --check
• PATH="$HOME/.nvm/versions/node/v24.15.0/bin:$PATH" npm run verify:pr

[Gracker]

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