Implement balance logging in watchdog worker

[callebtc]

Introduce functionality to store and log balances in the watchdog worker, enhancing monitoring of backend and keyset balances. Update balance retrieval methods to support this feature.

Todo:

• design killswitch mechanism
• tests
• ponder: backend.status() should return int and not float?

[codecov]

Codecov Report

Attention: Patch coverage is 82.72251% with 33 lines in your changes missing coverage. Please review.

Project coverage is 66.02%. Comparing base (23a706d) to head (2b2dc45).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
cashu/core/base.py	71.92%	16 Missing ⚠️
cashu/lightning/fake.py	54.54%	5 Missing ⚠️
cashu/mint/watchdog.py	94.82%	3 Missing ⚠️
cashu/lightning/clnrest.py	0.00%	2 Missing ⚠️
cashu/lightning/corelightningrest.py	0.00%	2 Missing ⚠️
cashu/wallet/cli/cli.py	33.33%	2 Missing ⚠️
cashu/lightning/lnd_grpc/lnd_grpc.py	0.00%	1 Missing ⚠️
cashu/lightning/lndrest.py	50.00%	1 Missing ⚠️
cashu/mint/crud.py	95.23%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #705      +/-   ##
==========================================
+ Coverage   65.67%   66.02%   +0.34%     
==========================================
  Files          88       84       -4     
  Lines        9513     9391     -122     
==========================================
- Hits         6248     6200      -48     
+ Misses       3265     3191      -74     

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