feat(backend): add standardized API response envelope

[GRACENOBLE]

Problem

API responses have no consistent shape. Some handlers return raw structs, others return ad-hoc maps. Clients (web, mobile) have to handle each endpoint differently.

Proposed shape

// success
{ "data": { ... } }

// error
{ "error": { "code": "NOT_FOUND", "message": "user not found" } }

Acceptance criteria

• Define Response[T]" and ErrorResponsetypes in a shared package (e.g.internal/transport/handlers/response.go`)
• Add helpers: JSON(c, data)" and JSONError(c, status, code, message)`
• Update all existing handlers to use the helpers
• Document the envelope format in backend/docs/

[GRACENOBLE]

Implemented and merged via PR #65.