feat(portal-openbaar): surface the citizen reference in the public register (refs #78)

The BFF's public view exposes id/status/reference (never bsn/naam) and searches by
id or reference; the openbaar register's Referentie column and search now show the
reference the citizen saw on submit. api-client + openapi.json regenerated.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

services/bff/openapi.json

@@ -67,7 +67,8 @@
       "OpenbaarEntry": {
         "required": [
           "id",
-          "status"
+          "status",
+          "reference"
         ],
         "type": "object",
         "properties": {
@@ -76,6 +77,12 @@
           },
           "status": {
             "type": "string"
+          },
+          "reference": {
+            "type": [
+              "null",
+              "string"
+            ]
           }
         }
       },
@@ -101,4 +108,4 @@
       "name": "Bff.Api"
     }
   ]
-}
+}