`EdgeIdentityViewSet` composite-key lookup

[khvn26]

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.