feat(enterprise): add tenant tombstone purge flow

[Gracker]

Summary

• Complete enterprise multi-tenant §0.5.5 tenant tombstone and purge flow.
• Add /api/tenant/tombstone and /api/tenant/purge with confirm-tenant guard, admin/delete permission checks, 7-day purge window, async purge job polling, and purge proof hash.
• Block tenant mutations after tombstone for trace uploads and agent analyze requests, and block purge while active runs or trace-processor leases remain.
• Document the completed slice in docs/features/enterprise-multi-tenant/README.md.

Verification

• cd backend && PATH="$HOME/.nvm/versions/node/v24.15.0/bin:$PATH" npx jest --runInBand --forceExit src/routes/__tests__/enterpriseTenantRoutes.test.ts src/routes/__tests__/enterpriseTraceMetadataRoutes.test.ts src/routes/__tests__/agentRoutesRbac.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

[Gracker]

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