Fix web UI session files not being deleted on shutdown

[HarshitR2004]

Description

The session cleanup was not working after shutting down the web UI, even when the configuration had:

• web_use_output_dir = False
• keep_sessions = False

This issue was caused by the deprecated @app.on_event("shutdown") in FastAPI (FastAPI Startup and Shutdown Events Documentation)

Solution

# Add signal handlers for graceful shutdown
def handle_shutdown(signum, frame):
    app_state.server.should_exit = True

signal.signal(signal.SIGINT, handle_shutdown)
signal.signal(signal.SIGTERM, handle_shutdown)

# Start the web server
try:
    app_state.loop.run_until_complete(app_state.server.serve())
finally:
    if (
        not app_state.web_settings["keep_sessions"]
        and not app_state.web_settings["web_use_output_dir"]
    ):
        sessions_dir = Path(get_spotdl_path() / "web/sessions")
        logger.info("Removing sessions directories")
        if sessions_dir.exists():
            shutil.rmtree(sessions_dir)

1. Signal-based shutdown: Handles SIGINT and SIGTERM to trigger a graceful server exit.
2. Server run: Starts the Uvicorn server asynchronously.
3. Guaranteed cleanup: Uses a finally block to always delete the web/sessions directory if keep_sessions and web_use_output_dir are False.

Note: The existing shutdown method (@router.on_event("shutdown")) has been retained for backward compatibility.

Related Issue

Closes #2272

Testing

Steps:

1. Started the server and launched the web UI with:
   • web_use_output_dir = False
   • keep_sessions = False
2. Downloaded a song and verified its existence in the web/sessions directory.
3. Shut down the server using Ctrl + C.
4. Confirmed the log message: "Removing sessions directories".
5. Verified that all session folders were deleted.

Types of Changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• My code follows the code style of this project
• My change requires a change to the documentation
• I have updated the documentation accordingly
• I have added tests to cover my changes
• All new and existing tests passed
• I have read the CONTRIBUTING document

[Copilot]

Pull Request Overview

This PR fixes a bug where session files were not being cleaned up when shutting down the web UI, despite having web_use_output_dir = False and keep_sessions = False configured. The issue stemmed from the deprecated FastAPI @app.on_event("shutdown") not executing properly.

Key Changes:

• Added signal handlers (SIGINT, SIGTERM) for graceful shutdown detection
• Wrapped server execution in try-finally block to guarantee cleanup
• Added explicit session directory removal logic in the finally block

---

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[Copilot]

The handle_shutdown function lacks a docstring explaining its purpose as a signal handler. Consider adding a docstring to document the parameters (signum, frame) and its role in graceful shutdown.

Suggested change

	def handle_shutdown(signum, frame):
	def handle_shutdown(signum, frame):
	"""
	Signal handler to gracefully shut down the web server.
	Args:
	signum (int): The signal number received (e.g., SIGINT, SIGTERM).
	frame (FrameType): The current stack frame (unused).
	Sets the server's should_exit flag to True, initiating a graceful shutdown.
	"""

[Silverarmor]

Ready for merge EXCEPT isort is failing. Please run all linting as instructed in CONTRIBUTING.md.