feat: update backend-ci.yml to use uv

Replaces pip install steps with astral-sh/setup-uv and uv sync --group ci.
tox and tox-uv are in the ci group; uv sync also installs the project
itself so everything tox needs is available. uv.lock ensures reproducible
installs in CI.

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

Author: feanil

.github/workflows/backend-ci.yml

@@ -28,16 +28,16 @@ jobs:
         with:
           python-version: ${{ matrix.python-version }}
 
-      - name: Install pip
-        run: pip install -r requirements/pip.txt
+      - name: Install uv
+        uses: astral-sh/setup-uv@v4
 
-      - name: Install Dependencies
-        run: pip install -r requirements/ci.txt
+      - name: Install CI dependencies
+        run: uv sync --group ci
 
       - name: Run Tests
         env:
           TOXENV: ${{ matrix.toxenv }}
-        run: tox
+        run: uv run tox
 
       - name: Run coverage
         if: matrix.python-version == '3.12' && matrix.toxenv == 'django52'