fix(admin): lazy-load request payloads

[mylukin]

Summary

• add lightweight payload meta/part APIs for request detail pages
• load request/response/upstream payload bodies only on demand in the admin detail UI
• add a short per-scope cache for /admin/api/memory/stats
• keep payload_retention_days at 3 days and bump release version to 0.25.10

Verification

• pnpm test -- apps/gateway/src/routes/admin/admin.test.ts apps/gateway/src/routes/admin/memory.test.ts apps/admin/src/routes/requests/requests.test.ts packages/core/src/store/store-contract.test.ts
• pnpm typecheck
• pnpm lint
• pnpm build