`EdgeIdentityViewSet` composite-key lookup

### Scope

- Extend `EdgeIdentityViewSet` to support lookup by `composite_key` (`{environment_api_key}_{identifier}`) in addition to the existing `identity_uuid` path. Implementation route: query DDB by primary key (`composite_key` is the table's partition key) and return the same retrieve serialiser shape.

### Acceptance criteria

- `GET /api/v1/environments/{api_key}/edge-identities/by-composite-key/{composite_key}/` (or equivalent route the API team prefers) returns the same payload as the existing `/{identity_uuid}/` retrieve.
- 404 for an unknown composite_key.
- 403 with the existing edge-identities permission when the user lacks env identity view permission.