fix: unmute members when their mute expires

[ToxicBiohazard]

Types of changes

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• Bugfix (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 not to work as expected).
• Documentation Update (if none of the other choices applies).

Proposed changes

Automatic unmute never ran. unmute_time and unban_time are stored as Unix seconds, but the scheduler compared them to a millisecond cutoff, so every mute and ban matched, including permanent bans of about 500 weeks. The job then slept inside schedule() until the furthest deadline, and unbans run before unmutes, so a long ban blocked every unmute.

/mute also started its unmute task without keeping a reference. On Python 3.13 the event loop only holds a weak reference, so that sleeping task could be garbage-collected and never fire.

This change:

• Compares deadlines in seconds and only selects records due within the next minute.
• Runs unban and unmute work in the background instead of awaiting it inside the one-minute loop.
• Keeps a strong reference to the unmute task created by /mute.

Checklist

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• I have read and followed the CONTRIBUTING.md
  doc.
• Lint and unit tests pass locally with my changes.
• I have added the necessary documentation (if appropriate).

Additional context

Covered by tests/src/cmds/automation/test_scheduled_tasks.py: a far-future ban does not block scheduling, and an expired mute is cleared while a mute still hours away is left in place.

Made with Cursor

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 69.36%. Comparing base (76b2f25) to head (24c09fe).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #194      +/-   ##
==========================================
+ Coverage   66.60%   69.36%   +2.75%     
==========================================
  Files          54       54              
  Lines        3177     3215      +38     
==========================================
+ Hits         2116     2230     +114     
+ Misses       1061      985      -76     

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