fix: install Maven 3.9.9 in CI — mvn was not in PATH on pi-runner

actions/setup-java@v4 installs the JDK but not Maven itself.
pi-runner does not have mvn pre-installed. Add an explicit curl
install step matching the approach used in backend/Dockerfile.

.gitea/workflows/ci.yml

@@ -37,6 +37,15 @@ jobs:
           java-version: '21'
           cache: maven
 
+      # actions/setup-java installs the JDK but not Maven.
+      # pi-runner has no pre-installed mvn — download the same version used
+      # in backend/Dockerfile to keep the build environment consistent.
+      - name: Install Maven 3.9.9
+        run: |
+          curl -fsSL https://archive.apache.org/dist/maven/maven-3/3.9.9/binaries/apache-maven-3.9.9-bin.tar.gz \
+            | tar -xz -C /opt
+          echo "/opt/apache-maven-3.9.9/bin" >> $GITHUB_PATH
+
       - name: Run tests
         working-directory: backend
         run: mvn -q -ntp clean verify