fix: improve backend process handling in launch script

[vcfgv]

📝 Pull Request Template

1. Related Issue

Closes # (issue number)

Type of Change (select one)

Bug Fix

3. Description

• Intermediate Shell Process: Invoking subprocess.Popen with shell=True spawns an intermediate shell process (e.g., /bin/sh) to execute the command. The FastAPI application runs as a child process of this shell. Consequently, the process object returned by Popen references the shell's PID, not the application's PID.

• Signal Propagation Failure: When a SIGINT (Ctrl+C) is issued, the signal is received by the shell. Most shells terminate immediately upon receiving this signal without forwarding it to their child processes.

• Orphaned Process Creation: The shell terminates successfully, causing the parent Python script to conclude that the subprocess has ended. However, the underlying FastAPI process continues to run in the background as an orphan process, resulting in a process leak.

4. Testing

• I have tested this locally.
• I have updated or added relevant tests.

5. Checklist

• I have read the Code of Conduct
• I have followed the Contributing Guidelines
• My changes follow the project's coding style