docs(design): propose elastickv admin dashboard for DynamoDB/S3 CRUD (#611)

Proposes an embedded admin UI (Go + React SPA) that reuses the existing
DynamoDB and S3 adapter handlers in-process, so no new write paths are
introduced. Covers API shape, SigV4-backed auth with read-only/full
roles, CSRF, phased rollout, testing strategy, and rejected
alternatives.

Author: bootjp