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>

Dockerfile

@@ -1,5 +1,5 @@
 # Stage 1: Build Vue frontend
-FROM node:20-alpine AS build-frontend
+FROM node:22-alpine AS build-frontend
 WORKDIR /build
 COPY frontend/package.json frontend/package-lock.json* ./
 RUN npm install -g npm@latest --quiet && npm install