Fix CI for Gitea Actions runner: use Python container images

Replace actions/setup-python@v5 (skipped by act runner) with
python:3.12-slim container images so Python is available directly.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.gitea/workflows/ci.yml

@@ -8,11 +8,10 @@ on:
 jobs:
   lint:
     runs-on: ubuntu-latest
+    container:
+      image: python:3.12-slim
     steps:
       - uses: actions/checkout@v4
-      - uses: actions/setup-python@v5
-        with:
-          python-version: "3.12"
       - run: pip install ruff black
       - name: Check formatting
         run: black --check src/ tests/ frontends/
@@ -21,24 +20,19 @@ jobs:
 
   typecheck:
     runs-on: ubuntu-latest
+    container:
+      image: python:3.12-slim
     steps:
       - uses: actions/checkout@v4
-      - uses: actions/setup-python@v5
-        with:
-          python-version: "3.12"
       - run: pip install mypy
       - run: mypy src/
 
   test:
     runs-on: ubuntu-latest
-    strategy:
-      matrix:
-        python-version: ["3.11", "3.12", "3.13"]
+    container:
+      image: python:3.12-slim
     steps:
       - uses: actions/checkout@v4
-      - uses: actions/setup-python@v5
-        with:
-          python-version: ${{ matrix.python-version }}
       - name: Install dependencies
         run: pip install -e ".[dev]"
       - name: Run tests