feat(enterprise): add tenant export bundle

[Gracker]

Summary

• add GET /api/export/tenant for tenant-scoped JSON export bundles
• include tenant identity proof, trace manifests, report HTML/JSON, sessions/runs/turns, memory entries, audit subset, and provider metadata
• exclude trace file bodies and redact provider secrets / secret-like JSON keys
• compute stable JSON SHA256 and audit tenant.exported
• mark enterprise multi-tenant README §0.5.4 complete with implementation notes

Verification

• cd backend && PATH="$HOME/.nvm/versions/node/v24.15.0/bin:$PATH" npx jest --runInBand --forceExit src/routes/tests/enterpriseTenantExportRoutes.test.ts
• cd backend && PATH="$HOME/.nvm/versions/node/v24.15.0/bin:$PATH" npx jest --runInBand --forceExit src/routes/tests/enterpriseTenantExportRoutes.test.ts src/routes/tests/enterpriseReportRoutes.test.ts src/services/tests/enterpriseDb.test.ts src/services/tests/enterpriseRepository.test.ts src/services/tests/enterpriseSchema.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。