feat(bff): committed OpenAPI contract + drift guard (refs #8)

Document typed responses (202 SubmitAccepted / 400 / 401 on self-service; 200
OpenbaarEntry[] on openbaar) so the generated spec carries real schemas for S-08's
client. A document transformer clears the auto-populated servers block so the spec
is host-independent and deterministic. Commit services/bff/openapi.json and add a
test asserting it matches the served /openapi/v1.json (fails on drift).

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

services/bff/openapi.json

@@ -0,0 +1,104 @@
+{
+  "openapi": "3.1.1",
+  "info": {
+    "title": "Bff.Api | v1",
+    "version": "1.0.0"
+  },
+  "paths": {
+    "/self-service/registrations": {
+      "post": {
+        "tags": [
+          "Bff.Api"
+        ],
+        "responses": {
+          "202": {
+            "description": "Accepted",
+            "content": {
+              "application/json": {
+                "schema": {
+                  "$ref": "#/components/schemas/SubmitAccepted"
+                }
+              }
+            }
+          },
+          "400": {
+            "description": "Bad Request"
+          },
+          "401": {
+            "description": "Unauthorized"
+          }
+        }
+      }
+    },
+    "/openbaar/register": {
+      "get": {
+        "tags": [
+          "Bff.Api"
+        ],
+        "parameters": [
+          {
+            "name": "q",
+            "in": "query",
+            "schema": {
+              "type": "string"
+            }
+          }
+        ],
+        "responses": {
+          "200": {
+            "description": "OK",
+            "content": {
+              "application/json": {
+                "schema": {
+                  "type": "array",
+                  "items": {
+                    "$ref": "#/components/schemas/OpenbaarEntry"
+                  }
+                }
+              }
+            }
+          }
+        }
+      }
+    }
+  },
+  "components": {
+    "schemas": {
+      "OpenbaarEntry": {
+        "required": [
+          "id",
+          "status"
+        ],
+        "type": "object",
+        "properties": {
+          "id": {
+            "type": "string"
+          },
+          "status": {
+            "type": "string"
+          }
+        }
+      },
+      "SubmitAccepted": {
+        "required": [
+          "registrationId",
+          "status"
+        ],
+        "type": "object",
+        "properties": {
+          "registrationId": {
+            "type": "string"
+          },
+          "status": {
+            "type": "string"
+          }
+        }
+      }
+    }
+  },
+  "tags": [
+    {
+      "name": "Bff.Api"
+    }
+  ]
+}