Epic: Backend Implementations and Parity

[niteshpurohit]

Summary

Covers concrete backend implementations, parity, and backend-specific validation.

Scope

• supported backend implementations
• persistence parity, backend validation, and backend comparison guidance
• explicit treatment of local backends versus shared/server backends

Acceptance Criteria

• backend implementation work is grouped separately from the shared contract epic
• backend tiering remains explicit: local InMemory|SQLite, shared/server Redis|Postgres|MySQL