ci: add backend unit test job to pipeline

Runs ./mvnw clean test in a dedicated job — no DB or S3 needed
since all tests use Mockito or WebMvcTest slices.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.gitea/workflows/ci.yml

@@ -38,6 +38,26 @@ jobs:
           name: unit-test-screenshots
           path: frontend/test-results/screenshots/
 
+  # ─── Backend Unit & Slice Tests ───────────────────────────────────────────────
+  # Pure Mockito + WebMvcTest — no DB or S3 needed.
+  backend-unit-tests:
+    name: Backend Unit Tests
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: actions/setup-java@v4
+        with:
+          java-version: '21'
+          distribution: temurin
+          cache: maven
+
+      - name: Run backend tests
+        run: |
+          chmod +x mvnw
+          ./mvnw clean test
+        working-directory: backend
+
   # ─── E2E Tests ────────────────────────────────────────────────────────────────
   # Needs: PostgreSQL + MinIO (via docker-compose) + Spring Boot + SvelteKit dev server.
   # Test data is seeded by DataInitializer on first startup (admin user + e2e profile data).