Restore venv activation in test step, clear VPS cache

Cached venv has broken internal paths even when using venv/bin/python
directly. Clear caches and use . venv/bin/activate + python -m pytest
which correctly sets VIRTUAL_ENV for module resolution.

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

.gitea/workflows/deploy.yml

@@ -88,7 +88,8 @@ jobs:
       - name: Run tests with coverage
         run: |
           cd dashboard/backend
-          venv/bin/python -m pytest tests/ -v --timeout=60 \
+          . venv/bin/activate
+          python -m pytest tests/ -v --timeout=60 \
             --cov=. --cov-report=xml --cov-report=term \
             --junit-xml=test-results.xml \
             --cov-fail-under=49