feat: option to configure Flask listening address

[L2jLiga]

Added environment variable to configure gunicorn bind address, so it can be changed to [::] for IPv6-only / dual-stack environments

[gemini-code-assist]

Code Review

This pull request updates the Gunicorn bind address in supervisord.conf to support IPv6 and adds a trailing newline. Feedback suggests removing the single quotes from the bind address to prevent potential parsing errors and considering an environment variable for the bind address to ensure compatibility with systems where IPv6 is disabled.

[gemini-code-assist]

While binding to [::] is the standard way to support both IPv4 and IPv6 on modern Linux systems, there are two points to consider:

1. Compatibility: Gunicorn will fail to start if the host system has IPv6 completely disabled at the kernel level (e.g., via ipv6.disable=1). For a more robust implementation, consider making the bind address configurable via an environment variable to allow users to revert to 0.0.0.0 if needed.
2. Quotes: The single quotes around [::]:8000 are unnecessary in a supervisord.conf command string and might be passed literally to Gunicorn depending on the Supervisor version's parsing logic, which could lead to binding errors. It is safer to remove them.

command=/usr/local/bin/gunicorn --bind [::]:8000 --workers 1 --threads 4 --worker-class gthread --keep-alive 5 --timeout 300 app:app