Expose semanticId in API v3 work package endpoints

Adds the computed semanticId property to the HAL representer,
SQL collection representer, and schema representer. The property is
gated behind the semantic_work_package_ids feature flag and returns the
value from WorkPackage#identifier. Includes OpenAPI docs
and the translation key for the schema name.

docs/api/apiv3/components/schemas/work_package_model.yml

@@ -12,6 +12,12 @@ allOf:
         description: Work package id
         readOnly: true
         minimum: 1
+      semanticId:
+        type: string
+        description: |-
+          The project-based semantic identifier for the work package (e.g. PROJ-42).
+          Only present when semantic mode is enabled.
+        readOnly: true
       lockVersion:
         type: integer
         description: The version of the item as used for optimistic locking