Survival curve #14

Why

Judging priority was continuation. We should measure it.

What to build

• A survival_points table and a survival job and subcommand. Each run adds a point: day, repos_total, repos_with_commit_since_deadline, repos_with_commit_last_7d and repos_live_on_mainnet (null until the graduation watcher exists). Commit data comes from github/history.py.
• .github/workflows/survival.yml: Monday 06:00 UTC, runs the job with repository secrets for the database and GitHub token.
• An API route and a survival chart on the home page, based on the pattern in components/charts/timeline-area.tsx. The chart stays hidden until there are two points.

Acceptance criteria

• The workflow runs green on a fork with a token.
• The chart is hidden with fewer than 2 points and shown after.

See CONTRIBUTING.md for conventions.