Update Moxfield API integration according to new authentication method

[ndepaola]

Description

• Context here: https://bsky.app/profile/moxfield.com/post/3lbvjdfcyh22a
• Resolves Moxfield links no longer importing #265
• Implemented by adding a new environment variable, MOXFIELD_SECRET, which is passed to Moxfield API requests as the User-Agent header
• If MOXFIELD_SECRET is not set, the Moxfield integration will be turned off
• I've added my Moxfield secret to the repo so CI tests can exercise the integration

Checklist

• I have installed pre-commit and installed the hooks with pre-commit install before creating any commits.
• I have updated any related tests for code I modified or added new tests where appropriate.
• I have manually tested my changes as follows:
  • Checked the Moxfield integration is turned off if no secret is set
  • Checked Moxfield decklists can be imported if the secret is valid
• I have updated any relevant documentation or created new documentation where appropriate.
  • Will do this separately after merging - github wikis are managed as their own git repos