chore: ci: add workflow for backend

Signed-off-by: Christoph Heiss <christoph@c8h4.io>

.forgejo/workflows/backend.yaml

@@ -0,0 +1,45 @@
+---
+name: backend
+
+on:
+  push:
+    branches:
+      - main
+  pull_request:
+    paths:
+      - 'backend/**'
+      - .forgejo/workflows/backend.yaml
+
+env:
+  MAVEN_VERSION: '3.9.8'
+  MAVEN_OPTS: '-Dmaven.repo.local=.m2'
+  MAVEN_ARGS: '--batch-mode --errors --fail-at-end --show-version'
+
+jobs:
+  test:
+    runs-on: debian-latest
+    steps:
+      - name: checkout repository
+        uses: actions/checkout@v4
+
+      - name: set up java
+        uses: https://github.com/actions/setup-java@v4
+        with:
+          java-version: '21'
+          distribution: temurin
+          cache: maven
+          cache-dependency-path: backend/pom.xml
+
+      - name: set up maven
+        run: |
+          curl -L "https://repo.maven.apache.org/maven2/org/apache/maven/apache-maven/$MAVEN_VERSION/apache-maven-$MAVEN_VERSION-bin.tar.gz" | tar -vxzC /opt
+          echo "/opt/apache-maven-$MAVEN_VERSION/bin" >>$GITHUB_PATH
+
+      - name: run tests
+        working-directory: backend
+        run: |
+          mvn \
+            clean \
+            org.jacoco:jacoco-maven-plugin:prepare-agent \
+            test \
+            org.jacoco:jacoco-maven-plugin:report