feat: graceful shutdown (#58)

[edehvictor]

Summary

This PR implements a robust graceful shutdown mechanism to ensure the backend safely drains in-flight requests and closes database connections on SIGTERM and SIGINT signals, preventing 502 errors during deployments or restarts.

Fixes #58

Changes Made

• Signal Handlers: Registered listeners for SIGTERM and SIGINT inside src/index.ts.
• Load Balancer Draining: Calls server.close() immediately to stop accepting new HTTP requests.
• In-Flight Request Tracking: Implemented raw TCP socket tracking using a Set (activeConnections) to ensure critical billing and settlement requests are fully processed before the server exits.
• Database Teardown: Extracted closeDb() in src/db/index.ts and integrated it into the shutdown sequence to safely close the SQLite connection.
• Timeout Safety Net: Added a 30-second maximum shutdown timeout (TIMEOUT_MS) to force-exit the process if connections hang indefinitely.

Testing

• Verified server stops accepting new connections immediately on CTRL+C.
• Verified the event loop waits for active connections to drop to 0.
• Verified the database closes successfully before process.exit(0).