Store proofs used for paying a lightning invoice

[sihamon]

Closes #66.

[codecov]

Codecov Report

Patch coverage: 55.55% and project coverage change: -0.33% ⚠️

Comparison is base (87c0adc) 57.15% compared to head (3c334d2) 56.83%.
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #311      +/-   ##
==========================================
- Coverage   57.15%   56.83%   -0.33%     
==========================================
  Files          43       43              
  Lines        4010     4110     +100     
==========================================
+ Hits         2292     2336      +44     
- Misses       1718     1774      +56     

Files Changed	Coverage Δ
cashu/mint/ledger.py	28.11% <0.00%> (-0.13%)	⬇️
cashu/mint/crud.py	69.33% <60.00%> (-0.67%)	⬇️
cashu/mint/migrations.py	100.00% <100.00%> (ø)

... and 2 files with indirect coverage changes

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[AngusP]

Sorry not a very useful review 😅 -- the msat vs sat 'issue' is more general than this PR but seemed worth pointing out anyway

[AngusP]

Perhaps worth the explicit 'cast' to an int to indicate that's what's going to happen DB-side anyway?

Suggested change

	invoice.amount_msat / 1000,
	int(invoice.amount_msat / 1000),

[AngusP]

Storing an amount as a big_int, but it is converted from msats to sats above with / 1000, so any sub-sat amount is going to be truncated off... Probably OK, but a potential source of problems?