ADM-OPERATION-500 - Return a single Operation by ID

GET https://api.djust.example.com/admin/operations/{operationId}

Retrieves a single Operation visible to the caller based on its ID.

Feature flag
- Available only when OPERATIONS is ON (otherwise 403).

Access control
- dj-client = OPERATOR → can access any Operation.
- dj-client = ACCOUNT → can access the Operation only if:
- ownerId = callerUserId
- AND OPERATIONS_READ right is still granted.

Error cases
Operation not found or not accessible → 403 or 404 depending on context.