Node.js unhandled promise rejection crashing the process

Node.js process exits with UnhandledPromiseRejectionWarning or crashes on unhandled promise rejections.

Since Node 15+, unhandled rejections crash by default. Fixes: (1) Always add .catch() or use try/catch with await. (2) Add global handler: process.on('unhandledRejection', handler). (3) In Express: use express-async-errors or wrap async handlers. (4) Check for missing awaits in tests.

An unhandled rejection means an error in async code was not caught. Node treats this as a bug because silently swallowing errors leads to undefined behavior.