ADM-OPERATOR-USER-251 - Replace Operator User Stores

Overview
Replaces the list of stores assigned to the operator user identified by operatorUserId.

User scoping
Admin endpoint. Only operator users can call this endpoint (dj-client: OPERATOR).

Store scoping
The effective store context is derived from the dj-store and dj-store-view headers when provided. This context does not impact store assignment.

Replacement rules
Store access is always explicit:
- If the request body contains an empty array: the operator user has no store access (restricted from all stores).
- If the request body contains store external IDs: the operator user is restricted to those stores only.
- Duplicates in the request are ignored.

Errors
Returns validation errors (400), authentication errors (401), authorization errors (403), not found (404), and unexpected server errors (500).