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 50.00000% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 75.33%. Comparing base (ce07276) to head (6acb3c1).
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
cashu/mint/ledger.py	11.11%	8 Missing ⚠️
cashu/mint/router.py	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1002      +/-   ##
==========================================
- Coverage   75.41%   75.33%   -0.09%     
==========================================
  Files         110      110              
  Lines       11798    11812      +14     
==========================================
+ Hits         8897     8898       +1     
- Misses       2901     2914      +13     

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[a1denvalu3]

Needs to be rebased

[KvngMikey]

LGTM