docs: add CLAUDE.md with repo-specific guidance

[EricGrill]

Summary

• Adds CLAUDE.md at the repo root so future Claude Code sessions can come up to speed without re-deriving project conventions from scratch.

What's captured

Only the non-obvious stuff — not things a future instance could trivially discover with ls or by reading package.json:

• Four-app monorepo shape — backend/ (main), web/ (React dashboard), mobile/ (Expo), and frontend/ (single static HTML page served by the backend, distinct from web/).
• .env at repo root, not backend/ — server.js resolves it two levels up, which is easy to get wrong.
• Auth middleware pattern — requireAuth(minRole) is registered as a standalone middleware before the router so GETs stay public; AUTH_ENABLED auto-flips off when no keys are set (dev mode).
• Migration convention — idempotent try { ALTER TABLE ... } catch {} appended to getDb() (see existing pay_rate_sats / overtime_multiplier additions).
• Simulation seams — lib/liquid.js is the designated Phase-2 GDK swap point; lib/lightning.js auto-enters DEMO_MODE when LNbits keys are placeholder/absent.
• Cron registration is gated on require.main === module so tests/imports don't schedule jobs.
• Test quirks — NODE_ENV=test short-circuits both rate limiters; running a single test uses node --test --test-name-pattern='<regex>' test/api.test.js.

Test plan

• Verify CLAUDE.md renders correctly on GitHub
• Confirm commands listed (npm run dev, npm run seed, npm test, web lint/build, mobile start) match the actual package.json scripts

🤖 Generated with Claude Code