Enable Fly.io-compatible runtime and container startup

[Copilot]

This app was wired for local Docker Compose assumptions (fixed port + implicit local service topology), which blocks Fly.io deployment. This change makes runtime/network configuration platform-driven while preserving local defaults.

• Runtime port binding

  • Updated the server to bind to process.env.PORT with a local fallback, so Fly can inject the listen port at runtime.

• Container boot behavior

  • Added an explicit Docker startup command so the image reliably launches the Node process when deployed.

• Fly deployment configuration

  • Added fly.toml with http_service.internal_port = 3000 and PORT=3000 to align Fly routing with the containerized app.

• Database configuration model

  • Kept DB connectivity env-driven (DB_HOST/DB_PORT/DB_USER/DB_PASSWORD/DB_NAME, etc.) with local defaults intact, so Compose-based local development still works while Fly can point to external MySQL.

// app/app.js
const PORT = process.env.PORT || 3000;
app.listen(PORT, "0.0.0.0", () => {
  console.log(`Server running on port ${PORT}`);
});