feat(wallet): add efficient recovery

[a1denvalu3]

Summary

• implement efficient wallet recovery proposed in cashubtc/nuts#342
• add encrypted d_gap metadata to blinded messages and signatures
• persist and restore d_gap at the mint and advertise NUT-342 support
• calculate recovery gaps from unspent and pending proofs within the same keyset
• recover using windowed binary-search probes with NUT-13 fallback

Implementation notes

Binary-search probes use the legacy recovery batch size as their window size. This tolerates isolated derivation gaps while keeping the strategy wallet-defined and wire-compatible.

The wallet reuses the derivation counter already stored in each proof's derivation_path; no wallet schema migration is needed. The mint adds a nullable d_gap column to promises.

Testing

• poetry run ruff check .
• poetry run mypy cashu --check-untyped-defs
• focused efficient-recovery, wallet restore, mint database, and auth tests

Review considerations

• draft specification is still under discussion
• includes a mint database migration
• includes AES-128-GCM recovery metadata keyed from the deterministic blinding factor
• windowed binary search tolerates gaps within a probe window; wallets fall back to NUT-13 when the search assumptions cannot be established

[codecov]

Codecov Report

❌ Patch coverage is 83.33333% with 21 lines in your changes missing coverage. Please review.
✅ Project coverage is 74.65%. Comparing base (a935eb9) to head (230596b).
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
cashu/wallet/wallet.py	75.60%	20 Missing ⚠️
cashu/core/nuts/nut342.py	95.45%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1081      +/-   ##
==========================================
+ Coverage   74.54%   74.65%   +0.11%     
==========================================
  Files         115      116       +1     
  Lines       13195    13310     +115     
==========================================
+ Hits         9836     9937     +101     
- Misses       3359     3373      +14     

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[a1denvalu3]