Release v2.15.1 — fix CI: move supertest to backend/

v2.15.0 typecheck passed locally but failed in CI because supertest was
installed at the workspace root instead of in backend/package.json.
TypeScript resolves up the directory tree, so local tests found it via
root node_modules, but CI installs each sub-package independently.

Move both supertest and @types/supertest to backend/package.json.
No code changes — release purely to unblock CI.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: idemerge

CHANGELOG.md

@@ -4,6 +4,11 @@ All notable changes to this project will be documented in this file.
 
 The format is based on [Keep a Changelog](https://keepachangelog.com/), and this project adheres to [Semantic Versioning](https://semver.org/).
 
+## [2.15.1] - 2026-05-15
+
+### Fixed
+- CI: `supertest` and `@types/supertest` moved from root `package.json` to `backend/package.json`. Local tests passed because TypeScript resolves up the directory tree to root `node_modules`, but CI installs each sub-package independently and could not find the module. Type check now passes in CI
+
 ## [2.15.0] - 2026-05-15
 
 ### Changed

VERSION

@@ -1 +1 @@
-2.15.0
+2.15.1