Bootstrap copied BFF and local accounts with public Sendico contracts (#34)

backend/contracts/sendico/api/response/error.yaml

@@ -0,0 +1,24 @@
+components:
+  schemas:
+    ApiError:
+      description: Standard API error envelope with optional debugging details.
+      type: object
+      additionalProperties: false
+      required:
+        - code
+        - error
+        - source
+      properties:
+        code:
+          description: Application-specific numeric error code.
+          type: integer
+          format: int32
+        error:
+          description: Human-readable error summary.
+          type: string
+        source:
+          description: Component or subsystem that produced the error.
+          type: string
+        details:
+          description: Optional detailed message providing additional failure context.
+          type: string