Initial commit: add .gitignore and README

docs/api-error-format.md

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+# API error response format
+
+All API errors use a consistent JSON body:
+
+```json
+{
+  "error": "Human-readable message",
+  "code": "UNAUTHORIZED",
+  "details": {}
+}
+```
+
+- **error** (string): Message for clients and logs.
+- **code** (string, optional): Machine-readable code. One of `BAD_REQUEST`, `UNAUTHORIZED`, `FORBIDDEN`, `NOT_FOUND`, `CONFLICT`, `INTERNAL_ERROR`.
+- **details** (object, optional): Extra data (e.g. validation errors under `details` when `code` is `BAD_REQUEST`).
+
+HTTP status matches the error (400, 401, 403, 404, 409, 500). The OpenAPI spec references the `ApiError` schema in `components.schemas`.