add: pipeline for tests and linter

.github/workflows/ci.yml

@@ -0,0 +1,33 @@
+name: CI Pipeline
+
+on:
+  push:
+    branches: [ "main" ]
+  pull_request:
+    branches: [ "main" ]
+
+jobs:
+  lint-and-test:
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Code auschecken
+        uses: actions/checkout@v4
+
+      - name: Java 21 einrichten
+        uses: actions/setup-java@v4
+        with:
+          distribution: 'temurin'
+          java-version: '21'
+
+      - name: Gradle Rechte vergeben
+        run: chmod +x gradlew
+
+      - name: 1. Code-Style (Linter) prüfen
+        run: ./gradlew ktlintCheck
+
+      - name: 2. Tests ausführen
+        run: ./gradlew test
+
+      - name: 3. Backend bauen
+        run: ./gradlew build -x test