ci: limit CGAL build to -j2 and nice -n 19 to protect Pi web server

-j$(nproc) during CGAL+Eigen template compilation consumed ~1.5-2 GB
peak RAM on the Raspberry Pi CI runner, starving the Gitea web server.

Changes:
- CGAL build: -j$(nproc) → -j2 (halves peak memory, ~700 MB per process)
- Both builds: nice -n 19 (lowest CPU priority, web server keeps preemption)
- test-cgal container: hard memory cap --memory=1400m --memory-swap=1400m
  so the container is OOM-killed rather than taking down the whole system

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

.gitea/workflows/cpp-tests.yml

@@ -27,7 +27,7 @@ jobs:
         run: cmake -S code -B build -DCMAKE_BUILD_TYPE=Release
 
       - name: Build
-        run: cmake --build build --target conformallab_tests -j$(nproc)
+        run: nice -n 19 cmake --build build --target conformallab_tests -j$(nproc)
 
       - name: Run tests
         run: >
@@ -66,6 +66,7 @@ jobs:
     runs-on: eulernest
     container:
       image: git.eulernest.eu/conformallab/ci-cpp:latest
+      options: "--memory=1400m --memory-swap=1400m"
 
     steps:
       - uses: actions/checkout@v4
@@ -74,7 +75,7 @@ jobs:
         run: cmake -S code -B build -DWITH_CGAL_TESTS=ON -DCMAKE_BUILD_TYPE=Release
 
       - name: Build CGAL-Tests
-        run: cmake --build build --target conformallab_cgal_tests -j$(nproc)
+        run: nice -n 19 cmake --build build --target conformallab_cgal_tests -j2
 
       - name: Run CGAL-Tests
         run: >