Set up GitHub Actions CI for backend — build, test, lint

[GavT]

Summary

The backend has no CI pipeline. PRs should be gated on a clean build and passing tests.

What to create

File: .github/workflows/ci.yml (in fueller-backend repo)

name: Backend CI
on: [push, pull_request]
jobs:
  build-and-test:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v4
      - uses: actions/setup-java@v4
        with: { java-version: '17', distribution: 'temurin' }
      - uses: gradle/actions/setup-gradle@v3
      - run: ./gradlew test --stacktrace
      - uses: actions/upload-artifact@v4
        if: always()
        with:
          name: test-results
          path: backend/build/reports/tests/

Also add

• Docker build step to verify the image builds cleanly
• Secret FUEL_FINDER_CLIENT_ID and FUEL_FINDER_CLIENT_SECRET for integration test environment

Workstream: WS3 Phase 3A