mint: add seed decrypt

[callebtc]

Todo:

• migrate seeds in db to encrypted seeds with migration tool
• store seed in db only if encrypted: no
• decrypt seed from db

[codecov]

Codecov Report

Attention: 81 lines in your changes are missing coverage. Please review.

Comparison is base (30b6e8a) 69.30% compared to head (209b61b) 68.77%.

Files	Patch %	Lines
cashu/mint/decrypt.py	0.00%	76 Missing ⚠️
cashu/core/base.py	90.90%	2 Missing ⚠️
cashu/core/crypto/aes.py	95.23%	2 Missing ⚠️
cashu/core/settings.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #403      +/-   ##
==========================================
- Coverage   69.30%   68.77%   -0.53%     
==========================================
  Files          71       73       +2     
  Lines        6206     6348     +142     
==========================================
+ Hits         4301     4366      +65     
- Misses       1905     1982      +77     

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