feat(bff): documents endpoint accepts the base64 file and forwards it to the domain (refs #103)

The self-service documents endpoint takes { contentBase64, fileName, contentType }
as JSON (bsn from the token) and forwards it via IDomainClient.ProvideDocumentsAsync.
Regenerates openapi.json + the Angular client (postSelfServiceRegistrationsIdDocuments
now takes a ProvideDocumentsRequest body).

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

services/bff/openapi.json

@@ -76,6 +76,16 @@
             }
           }
         ],
+        "requestBody": {
+          "content": {
+            "application/json": {
+              "schema": {
+                "$ref": "#/components/schemas/ProvideDocumentsRequest"
+              }
+            }
+          },
+          "required": true
+        },
         "responses": {
           "204": {
             "description": "No Content"
@@ -228,6 +238,29 @@
           }
         }
       },
+      "ProvideDocumentsRequest": {
+        "required": [
+          "contentBase64"
+        ],
+        "type": "object",
+        "properties": {
+          "contentBase64": {
+            "type": "string"
+          },
+          "fileName": {
+            "type": [
+              "null",
+              "string"
+            ]
+          },
+          "contentType": {
+            "type": [
+              "null",
+              "string"
+            ]
+          }
+        }
+      },
       "SubmitAccepted": {
         "required": [
           "registrationId",