feat(deploy): add vps health checks

[Tuxedolphin]

Summary

• add GET /api/v2/health for VPS reverse proxy, uptime monitor, and Worker smoke checks
• allow GET through backend CORS and the Cloudflare Worker proxy
• add example systemd and nginx configs for a VPS-hosted FastAPI backend
• document the VPS verification path for a Cloudflare Pages frontend + VPS backend deployment

Verification

• uv run --python 3.13 pytest tests/integration/test_api_v2.py tests/integration/test_cors.py -q (48 passed)
• uv run --python 3.13 ruff check backend tests
• node --check deploy\\cloudflare-api-proxy\\src\\index.js
• npx --yes wrangler@latest deploy --dry-run --config deploy\\cloudflare-api-proxy\\wrangler.jsonc
• uv run --python 3.13 pytest tests/ (827 passed)
• pnpm lint
• pnpm test (23 passed)
• pnpm build
• pnpm audit --audit-level moderate
• git diff --check

Notes

• nginx.conf.example and linear-algebra-api.service.example are VPS templates; native nginx -t and systemd-analyze verify were not available in the Windows workspace.