Add employee-to-order assignments with FR-EM-3 conflict validation

Adds the Assignment core object (Order x Employee, date range) with full
CRUD, soft-delete/trash integration, and generated API clients.

Layers FR-EM-3 conflict checks onto assignment creation: qualification,
absence/availability, working-hours approximation, cross-order overlap,
and active-contract coverage. Each check's severity (Warning vs. Error)
is configurable at runtime via a new AssignmentValidationSettings
singleton and admin settings panel, instead of being hardcoded - lets
the business tune strictness per check without a redeploy.

Adds a live GET /api/assignments/check endpoint so the create-assignment
dialog can preview conflicts as the user picks employee/dates, before
they hit save, rather than only finding out after submitting.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

omsorgWeb/mitarbeiter-app/api-client-php/docs/Model/AssignmentValidationSettingsResponse.md

@@ -0,0 +1,13 @@
+# # AssignmentValidationSettingsResponse
+
+## Properties
+
+Name | Type | Description | Notes
+------------ | ------------- | ------------- | -------------
+**qualification_mode** | **string** |  | [optional]
+**absence_mode** | **string** |  | [optional]
+**working_hours_mode** | **string** |  | [optional]
+**overlap_mode** | **string** |  | [optional]
+**contract_mode** | **string** |  | [optional]
+
+[[Back to Model list]](../../README.md#models) [[Back to API list]](../../README.md#endpoints) [[Back to README]](../../README.md)