fix(firstleg): surface existing truck appointment

firstleg/src/main/resources/openapi/truck-appointments-v1.yaml

@@ -21,6 +21,7 @@ paths:
                       records: {type: array, items: {$ref: '#/components/schemas/DetailStableReferences'}}
     post:
       summary: Create a truck appointment for a released booking and its allocated real container
+      description: If the real container already has a non-terminal appointment, the server returns A0409 with the existing appointment number and instructs the operator to continue editing. This duplicate guard runs before booking-document validation; the database unique key provides the same response for concurrent creates.
       requestBody:
         required: true
         content:
@@ -28,6 +29,7 @@ paths:
             schema: {$ref: '#/components/schemas/SaveRequest'}
       responses:
         '200': {description: 'Created appointment', content: {application/json: {schema: {$ref: '#/components/schemas/DetailEnvelope'}}}}
+        '409': {description: 'A0409 - this container already has a non-terminal appointment; message includes its appointment number and asks the operator to continue editing'}
   /api/platform/v1/truck-appointments/{id}:
     parameters: [{$ref: '#/components/parameters/AppointmentId'}]
     get: