ADM-OPERATOR-USER-252 - Add Operator User Stores

Overview
Adds stores to the operator user identified by operatorUserId without removing existing assignments.

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.

Addition rules
- Stores in the request are added to the current assignment.
- Duplicates (already assigned stores) are ignored.
- Empty array in request body is a no-op.

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

Language
Credentials
Bearer
JWT
Response
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