Reduce build parallelism to prevent OOM in CI

backend/Dockerfile

@@ -77,11 +77,12 @@ RUN rm -rf build CMakeCache.txt
 RUN mkdir -p build
 
 # Build the application with RPATH set correctly
+# Use -j2 to limit memory usage (OOM with -j$(nproc) on constrained CI runners)
 RUN cd build && \
     cmake .. -DCMAKE_BUILD_TYPE=Release \
              -DCMAKE_INSTALL_RPATH="/usr/local/lib" \
              -DCMAKE_BUILD_WITH_INSTALL_RPATH=TRUE && \
-    cmake --build . -j$(nproc)
+    cmake --build . -j2
 
 # Copy config template (real config mounted at runtime via docker-compose)
 COPY config.json.example config.json