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

GET https://example.com/v1/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.