To implement HTTPS using Let's Encrypt and ngrok while modifying your Flask application, follow these structured steps:

## Step 1: Install and Set Up ngrok

1. **Download ngrok**:
   - Go to the [ngrok website](https://ngrok.com/download) and download the appropriate version for your OS.
   - Install ngrok by following the instructions on the site.

2. **Authenticate ngrok**:
   - Sign up for a free account at ngrok.com.
   - After signing up, you’ll receive an authtoken. Run the following command to set it up:
     ```bash
     ngrok config add-authtoken <your_auth_token>
     ```

## Step 2: Modify Flask to Accept HTTPS

Update your Flask application to use ngrok to tunnel HTTPS traffic:

1. **Change `run_flask()` to bind to HTTP**:
   This is so ngrok can handle the SSL termination. Update the endpoint by removing the `ssl_context`:
   ```python
   def run_flask():
       app.run(host="0.0.0.0", port=8888)
   ```

2. **Expose the Flask app via ngrok**:
   You can run ngrok to expose your Flask app by executing:
   ```bash
   ngrok http 8888
   ```
   This will give you a public HTTPS URL that tunnels to your local Flask app.

## Step 3: Modify the Development Environment (devenv.nix)

You’ll need to add the ngrok installation to your `devenv.nix` configuration, if it's not already installed.

1. **Edit `devenv.nix`**:

```nix
{ pkgs ? import <nixpkgs> {} }:

pkgs.mkShell {
  buildInputs = [
    pkgs.ngrok
  ];

  shellHook = ''
    # Set up ngrok to run automatically
    ngrok http 8888 &
  '';
}
```

This snippet sets up ngrok to run in the background whenever you start your development environment.

## Step 4: Implement the PKI Chain in `geturl.py`

To implement an HTTPS PKI chain, consider using the Let's Encrypt certificate. For local development, you can't use Let's Encrypt directly without a publicly reachable domain, but you can still prepare your code as if you're using secure connections.

1. **Modify `geturl.py`**:

Assuming your application logic has not changed, implement a function to fetch the ngrok URL:

```python
import os
import requests
import threading
import time

def get_ngrok_url():
    response = requests.get("http://localhost:4040/api/tunnels")
    tunnels = response.json()["tunnels"]
    for tunnel in tunnels:
        if tunnel["name"] == "http":
            return tunnel["public_url"]
    return None

def create_playlist():
    # Your existing code to create a playlist
    ...

def run_flask():
    app.run(host="0.0.0.0", port=8888)

def main():
    # Start Flask in a background thread
    flask_thread = threading.Thread(target=run_flask, daemon=True)
    flask_thread.start()

    # Wait for ngrok to be ready
    time.sleep(5)  # Allow some time for ngrok to initialize

    ngrok_url = get_ngrok_url()
    print(f"Ngrok URL: {ngrok_url}")

    print("Please authorize access via the following URL:", sp_oauth.get_authorize_url())

    while not os.path.exists("access_token.txt"):
        time.sleep(1)

    playlist_url = create_playlist()
    print(f"Playlist URL returned: {playlist_url}")

if __name__ == "__main__":
    main()
```

In this code:
- `get_ngrok_url()` fetches the ngrok tunnel URL dynamically.
- You can then safely use this URL to inform other services of your HTTPS endpoint.

## Summary

This setup will let you run a local Flask application using HTTPS through an ngrok tunnel. You process the playlist creation and output the URL correctly while working in a secure environment. If you later transition to production, you can directly implement Let's Encrypt for your domain, maintaining the same architecture with minimal changes.