Use uv run for backend tests instead of system pip install

The uv-managed Python is externally managed and rejects --system pip
installs. Use uv run --extra dev to handle venv creation, dependency
installation, and test execution in a single step.

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

.github/workflows/ci.yml

@@ -48,11 +48,8 @@ jobs:
           source "$HOME/.local/bin/env"
           echo "$HOME/.local/bin" >> "$GITHUB_PATH"
           uv python install 3.14
-      - name: Install dependencies
-        run: uv pip install --system --python 3.14 -e ".[dev]"
-        working-directory: backend
       - name: Run tests
-        run: uv run --python 3.14 pytest -q
+        run: uv run --python 3.14 --extra dev pytest -q
         working-directory: backend
         env:
           TEST_DATABASE_URL: postgresql+asyncpg://postgres:postgres@localhost:5433/nuzlocke_test