Run LMS stream processes in their own process group

[stamateviorel]

What does this change intend to accomplish?

LMS.disconnect() falls back to os.killpg(self.proc.pid, SIGKILL), but the spawned squeezelite/metadata processes were never made process-group leaders, so proc.pid is not a valid pgid — the killpg either fails or signals the parent's group. This starts both children with preexec_fn=os.setpgrp so each owns its own group, kills via os.killpg(os.getpgid(pid)), and tolerates ProcessLookupError when the process already exited (previously an exception escaped during stream teardown).

Running in production on our unit; stream stop/reassign no longer leaves orphaned squeezelite processes behind.

Checklist

• Have you tested your changes and ensured they work? (in production on a real AmpliPi)
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?
• If applicable, have you updated the CHANGELOG?
• Does your submission pass linting & tests? (python -m py_compile clean; happy to fix anything CI flags)

[codecov-commenter]

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Codecov Report

❌ Patch coverage is 0% with 10 lines in your changes missing coverage. Please review.
✅ Project coverage is 50.56%. Comparing base (7499989) to head (c66283a).
⚠️ Report is 103 commits behind head on main.

Files with missing lines	Patch %	Lines
amplipi/streams/lms.py	0.00%	10 Missing ⚠️
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Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1096      +/-   ##
==========================================
- Coverage   50.67%   50.56%   -0.11%     
==========================================
  Files          40       41       +1     
  Lines        7154     7509     +355     
==========================================
+ Hits         3625     3797     +172     
- Misses       3529     3712     +183     

Flag	Coverage Δ
unittests	50.56% <0.00%> (-0.11%)	⬇️

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