Use python -m pytest instead of pytest binary in CI

Venv symlinks break when cache is used across different environments
(VPS host vs NAS Docker). python -m pytest is resilient because it
loads pytest as a module, not via a shebang-based script.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.gitea/workflows/deploy.yml

@@ -89,7 +89,7 @@ jobs:
         run: |
           cd dashboard/backend
           . venv/bin/activate
-          pytest tests/ -v --timeout=60 \
+          python -m pytest tests/ -v --timeout=60 \
             --cov=. --cov-report=xml --cov-report=term \
             --junit-xml=test-results.xml \
             --cov-fail-under=49