Move gradlew, gradle/wrapper, and settings.gradle from backend/ to
the repo root so build commands run from the top-level directory.
This follows the standard multi-project Gradle layout where the build
tool lives alongside docker-compose.yml and all submodules.
- Move gradlew + gradle/wrapper/* from backend/ to repo root
- Move settings.gradle to root with rootProject.name and include 'backend'
- Create root build.gradle with convenience tasks: check, up, down, reset
- check task chains frontend lint → frontend test → backend check
- Update docker-compose.yml backend volume from ./backend:/app to .:/app
- Update backend.Dockerfile entrypoint to ./gradlew :backend:bootRun
- Update AGENTS.md: document ./gradlew check, up, down, reset
- Delete backend/settings.gradle (now at root)
- Add .gradle/ and build/ to .gitignore
- Add !gradle/wrapper/gradle-wrapper.jar exception (blocked by *.jar rule)
All 38 frontend tests and 33 backend tests pass via ./gradlew check.
