ADM-OPERATION-150 - Add variants to an Operation

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

Adds one or more variants to a specific Operation with optional quantity constraints.

Feature flag
- Requires OPERATIONS to be active.

Access control
- OPERATOR: can add variants to any Operation.
- ACCOUNT: can add variants only to Operations they own (ownerId = customerUserId) and if they have the OPERATIONS_ADD_VARIANTS right.

Store scoping
- Effective store is the dj-store header if provided, otherwise the tenant default store.
- ACCOUNT must be attached to the effective store; otherwise 403.

Quantity rules
- minQuantity is optional and can be 0.
- maxQuantity is optional. If provided, it must be greater than or equal to minQuantity.
- recommendedQuantity is optional. If provided, it must be between minQuantity and maxQuantity (if maxQuantity is defined).

Other constraints
- A variant cannot be added more than once to the same Operation.
- Only Operations in DRAFT status can accept new variants.