Fix Python jobs: use python:3.12-slim container instead of setup-python

actions/setup-python fails in act_runner because it tries to download
pre-built binaries from GitHub CDN which aren't available in this
environment. Using a Python container image directly is the correct
approach — same pattern as the Flutter jobs in fabled_app.

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

.forgejo/workflows/ci.yml

@@ -42,13 +42,11 @@ jobs:
   lint:
     name: Python 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"
-
       - name: Install ruff
         run: pip install ruff
 
@@ -58,14 +56,14 @@ jobs:
   test:
     name: Python tests
     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"
-          cache: "pip"
-          cache-dependency-path: pyproject.toml
+      # git is required by setuptools-scm during package build
+      - name: Install git
+        run: apt-get update -qq && apt-get install -y -qq git
 
       - name: Install package with dev deps
         run: pip install -e ".[dev]"