fix cv2 import in CI by installing the OpenCV shared libraries

The opencv-python wheel links against libGL at import time, so pytest
failed at collection with ImportError: libGL.so.1 on the runner image.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01JPUZks9z3Pi5oxw9GQ9znt

.gitea/workflows/validate.yml

@@ -36,6 +36,13 @@ jobs:
     steps:
       - uses: actions/checkout@v4
 
+      # The opencv-python wheel links against libGL at import time, so cv2
+      # cannot even be imported on the bare runner image.
+      - name: Install the OpenCV shared libraries
+        run: |
+          sudo apt-get update
+          sudo apt-get install -y --no-install-recommends libgl1 libglib2.0-0
+
       - uses: astral-sh/setup-uv@v5
         with:
           enable-cache: true