ci: consume shared ci-python:3.14 image; bump runtime to 3.14

Migrate to the FabledRulebook CI-Runner contract:

- .forgejo/workflows/ci.yml: all four jobs (typecheck/lint/test/build)
  now schedule on the `python-ci` runner label and run inside
  container.image: git.fabledsword.com/bvandeusen/ci-python:3.14
  (Python 3.14 + Node 24 + ruff + uv + Docker CLI). Dropped the inline
  uv install in the test job — uv is now baked into the image.
- Dockerfile: production runtime bumped to python:3.14-slim so test
  results stay representative against what we ship.
- ci-requirements.md: new file at repo root declaring image deps and
  per-job installs (per FabledRulebook ci-runners.md).
- infra/Dockerfile.runner-base: deleted. The in-repo runner base
  (Ubuntu 24.04 + Python 3.12 + Node 22) is superseded by the shared
  ci-python image. The runner-host deployment files
  (runner-compose.yml + act-runner-config.yml) stay as deployment-shape
  documentation; source of truth is the deployed config.
- docs/development.md: CI/CD + Runner sections refreshed.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Dockerfile

@@ -8,7 +8,8 @@ COPY frontend/ .
 RUN npm run build
 
 # Stage 2: Python runtime
-FROM python:3.12-slim AS runtime
+# Tracks CI image (ci-python:3.14) so test results stay representative.
+FROM python:3.14-slim AS runtime
 WORKDIR /app
 
 COPY pyproject.toml .