feat: implement async-melt functionality

[a1denvalu3]

Summary

• Added prefer_async field to PostMeltRequest.
• Implemented async_melt wrapper in Ledger to handle asynchronous lightning payments.
• Updated melt endpoint in router.py to route to async_melt when prefer_async is true.
• Updated wallet's melt method and CLI pay command to support async payment initiation.

[codecov]

Codecov Report

❌ Patch coverage is 59.09091% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 75.26%. Comparing base (7fd94b5) to head (f04ae44).
⚠️ Report is 5 commits behind head on main.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
cashu/mint/ledger.py	46.66%	8 Missing ⚠️
cashu/mint/router.py	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1002      +/-   ##
==========================================
- Coverage   75.26%   75.26%   -0.01%     
==========================================
  Files         100      100              
  Lines       11742    11758      +16     
==========================================
+ Hits         8838     8850      +12     
- Misses       2904     2908       +4     

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