Harden API against downed lightning node

[Copilot]

A downed LN node caused ECONNREFUSED to surface as an unhandled socket 'error' event, crashing the entire server process. A secondary bug in check_invoice_is_paid — the new Promise(async ...) anti-pattern — let ln_rpc rejections escape as unhandled promise rejections, also fatal in Node.js v15+.

Fixes

• src/index.js — Global process.on('uncaughtException') and process.on('unhandledRejection') handlers as last-resort safety nets; logs the error and keeps the process alive
• src/invoicing.js — Rewrote check_invoice_is_paid to eliminate the new Promise(async ...) anti-pattern:

  // Before: async executor errors silently escaped as unhandled rejections
  return new Promise(async (resolve, reject) => {
    const res = await this.ln_rpc(...)  // rejection here → unhandled
    resolve(res.error ? false : true)
  })
  
  // After: explicit error containment
  const rpc_promise = this.ln_rpc({ method: "waitinvoice", params })
    .then(res => res.error ? false : true)
    .catch((e) => { error("Error checking invoice..."); return undefined })
  return Promise.race([timeout_promise, rpc_promise])

  Also added timer.unref() on the timeout so it doesn't block process exit.
• src/router_config.js — Added missing try/catch to POST /ln-checkout/:id/check-invoice and PUT /admin/ln-checkout/new-verified-checkout; added Express error-handling middleware ((err, req, res, next)) at the end of config_router as a final catch-all for unhandled route errors

Tests

• Extended MockLNNodeController with simulate_node_down() / simulate_node_up() that causes connect_and_init to reject
• Added LN Flow — Downed LN node does not crash the server covering: verify-checkout with node down (→ 400), check-invoice with node down (→ 200 with paid: undefined), background poll with node down (no throw), and server still responsive after all failures

[danieldaquino]

Superseded by #40