[Wallet] DB optimization for faster payments

[callebtc]

Profiling results showed that calling wallet.load_proofs(), specifically the list comprehension [Proof(**dict(r)) for r in rows] in wallet/crud.py spends a lot of CPU time.

• gets rid of most unnecessary load_proofs calls
• skips db load if wallet.proofs is already populated in load_proofs()
• New argument load_proofs(reload=True) forces a db reload
• Removes most initial_balance and balance in wallet api
• Tests: Add fixture wallet to wallet_api_test

[codecov]

Codecov Report

Patch coverage: 83.33% and project coverage change: +0.05 🎉

Comparison is base (959cc00) 55.97% compared to head (0e7933c) 56.02%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #250      +/-   ##
==========================================
+ Coverage   55.97%   56.02%   +0.05%     
==========================================
  Files          42       42              
  Lines        3348     3343       -5     
==========================================
- Hits         1874     1873       -1     
+ Misses       1474     1470       -4     

Impacted Files	Coverage Δ
cashu/mint/app.py	0.00% <ø> (ø)
cashu/wallet/api/app.py	100.00% <ø> (ø)
cashu/wallet/api/responses.py	100.00% <ø> (ø)
cashu/wallet/api/router.py	77.83% <ø> (+0.99%)	⬆️
cashu/wallet/cli/cli_helpers.py	50.00% <0.00%> (ø)
cashu/wallet/cli/cli.py	49.71% <50.00%> (+0.13%)	⬆️
cashu/wallet/helpers.py	84.76% <100.00%> (+0.14%)	⬆️
cashu/wallet/wallet.py	81.83% <100.00%> (+0.16%)	⬆️

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