Mint: Recover pending melts at startup

[callebtc]

Recovers pending melt requests by checking the LN backends during startup.

[codecov]

Codecov Report

Attention: Patch coverage is 80.18018% with 22 lines in your changes are missing coverage. Please review.

Project coverage is 69.12%. Comparing base (1f1daca) to head (f09630b).

Files	Patch %	Lines
cashu/mint/ledger.py	85.71%	7 Missing ⚠️
cashu/lightning/corelightningrest.py	28.57%	5 Missing ⚠️
cashu/lightning/lnbits.py	62.50%	3 Missing ⚠️
cashu/lightning/lndrest.py	0.00%	2 Missing ⚠️
cashu/mint/router.py	0.00%	2 Missing ⚠️
cashu/mint/startup.py	0.00%	2 Missing ⚠️
cashu/core/errors.py	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #499      +/-   ##
==========================================
+ Coverage   67.89%   69.12%   +1.23%     
==========================================
  Files          76       76              
  Lines        6815     6876      +61     
==========================================
+ Hits         4627     4753     +126     
+ Misses       2188     2123      -65     

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