Migrate to free-threaded Python

[beer-psi]

There's potentially a few data races in the bot that are not thread safe (bot-level caches are the big one; these include prefixes, denylist, aliases and permissions IIRC). These will have to be taken care of.

There are no python:3.14t-alpine images yet (https://redirect.github.com/docker-library/python/issues/1082). That's also a problem.

Some libraries also need to be updated:

• selectolax (has free threaded wheels, but does not declare freethreaded compatibility)
• sqlalchemy (2.1 has freethreading)
• ciso8601 (took the liberty of making a free-threaded wheel since the free-threaded PR has been up for about 3 months at this point)

There's also probably some performance wins from putting gateway handling on another thread.