CI: custom runner-base image, Node 22, pip caching

- infra/Dockerfile.runner-base: new Ubuntu 24.04 image with Python 3.12
  and Node 22 LTS pre-installed, tagged py3.12-node22. Eliminates the
  ~3-4 min deadsnakes PPA install that ran on every test job.
- infra/runner-compose.yml: update ubuntu-latest label to use
  runner-base:py3.12-node22 instead of node:20-bullseye.
- .forgejo/workflows/ci.yml: remove Python 3.12 install steps (now in
  base image), add actions/cache for pip keyed on pyproject.toml hash,
  bump node-version 20 → 22.
- Dockerfile: bump frontend build stage node:20-alpine → node:22-alpine.

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

.forgejo/workflows/ci.yml

@@ -45,7 +45,7 @@ jobs:
 
       - uses: actions/setup-node@v6
         with:
-          node-version: "20"
+          node-version: "22"
           cache: "npm"
           cache-dependency-path: frontend/package-lock.json
 
@@ -75,16 +75,13 @@ jobs:
     steps:
       - uses: actions/checkout@v6
 
-      # The act runner base image ships Ubuntu 22.04 with Python 3.10.
-      # Our package requires 3.12, so install it from the deadsnakes PPA.
-      - name: Install Python 3.12
-        run: |
-          apt-get update -qq
-          apt-get install -y -qq software-properties-common
-          add-apt-repository -y ppa:deadsnakes/ppa
-          apt-get update -qq
-          apt-get install -y -qq python3.12 python3.12-dev python3.12-venv python3-pip git
-          python3.12 -m ensurepip --upgrade
+      # Python 3.12 is pre-installed in the runner-base image (py3.12-node22).
+      - name: Cache pip wheels
+        uses: actions/cache@v3
+        with:
+          path: ~/.cache/pip
+          key: pip-${{ hashFiles('pyproject.toml') }}
+          restore-keys: pip-
 
       - name: Create virtual environment
         run: python3.12 -m venv /opt/venv