feat: NUT-12 deterministic DLEQ nonce derivation and storage optimization

[a1denvalu3]

This PR implements the NUT-12 deterministic DLEQ nonce derivation (rejection-sampled HMAC-SHA256) as specified in the merged spec pull request: cashubtc/nuts#368

Summary of Changes:

1. Deterministic Nonce Derivation:

   • Isolated the rejection-sampled HMAC-SHA256 nonce derivation into a standalone helper function deterministic_dleq_nonce inside cashu/core/crypto/b_dhke.py.
   • Updated step2_bob_dleq to utilize the helper when p_bytes is not explicitly provided.
   • Retained full backward compatibility for existing tests/usage that supply a mock nonce scalar.

2. Performance & Storage Optimization:

   • Avoided redundant point multiplication of $C'$ by computing it once at the start of step2_bob_dleq and passing it to the helper function.
   • Optimized promise storage to stop saving the e and s DLEQ parameters in the database (dleq_e and dleq_s columns are now set to empty string for new promises, saving substantial storage space for active mints).
   • Annotated existing promises table schema definitions in cashu/mint/migrations.py to document the deprecation of the unused DLEQ columns.

3. On-the-fly Reconstruction:

   • Modified the mint's /restore and get_melt_quote endpoints to dynamically reconstruct missing DLEQ proof values on-the-fly using the mint's private keys.
   • Supports seamless backward compatibility: historical random-nonce proofs continue to load normally, while new promises have their proofs generated dynamically.

4. Testing:

   • Added the official NUT-12 deterministic nonce test vectors in tests/test_crypto.py.
   • Verified that all mint and wallet restore test suites pass flawlessly.

[codecov]

Codecov Report

❌ Patch coverage is 63.41463% with 15 lines in your changes missing coverage. Please review.
✅ Project coverage is 75.02%. Comparing base (13ba163) to head (7a281f5).
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
cashu/mint/ledger.py	38.09%	13 Missing ⚠️
cashu/core/crypto/b_dhke.py	90.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1062      +/-   ##
==========================================
+ Coverage   74.79%   75.02%   +0.23%     
==========================================
  Files         111      111              
  Lines       12413    12452      +39     
==========================================
+ Hits         9284     9342      +58     
+ Misses       3129     3110      -19     

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