Bootstrap CodePress app-server start recipe

[codepress-dev]

Summary

• Adds a Dockerfile (two-stage: Node builds the React frontend into server/static/build, then a Python 3.6 image installs the Django backend via pipenv and serves the built app) plus a .dockerignore so future builds don't get polluted by host-side node_modules/.venv.
• Adds .codepress/start-app-server/recipe.json and .claude/skills/start-app-server/SKILL.md — a machine-readable and human-readable fast-path so future sessions can start the app server in one shot without re-discovering the stack.
• No app code was changed. The server was built, started, and validated with a real GET / request (HTTP 200, serving the React app) before anything was committed.

Test plan

• docker build succeeds for the generated Dockerfile
• Container starts and passes health check
• GET / returns HTTP 200 with the built React app HTML
• Repo maintainer spot-checks the Dockerfile/recipe and merges

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