feat: add payment-method abstraction

[a1denvalu3]

Summary

• add a typed, allowlisted payment-method plugin registry with BOLT11 as the built-in adapter
• generalize mint quote, melt quote, settlement, feature discovery, wallet APIs, persistence, and CLI method selection
• retain the existing /v1/.../bolt11 endpoints, defaults, wire fields, and legacy wallet tables for older clients
• add method-specific metadata storage and safe response projection so internal quote fields are never exposed as plugin extras

This establishes the abstraction boundary for methods such as BOLT12 or on-chain payments without adding or advertising a new production payment rail.

Compatibility

• existing BOLT11 routes and public wallet methods remain available
• BOLT11 remains the default method
• legacy BOLT11 quote tables remain canonical for existing clients
• new database changes are additive
• method-specific request/response data is validated by the selected plugin

Validation

• BOLT11 mint/melt: 41 passed, 1 skipped
• wallet: 32 passed, 1 skipped
• payment-method regression tests: 7 passed
• Ruff: passed
• git diff --check: passed

make check still reports two unrelated existing Optional-path mypy errors in cashu/mint/management_rpc/management_rpc.py.

Review notes

This includes additive schema and protocol-boundary work, so it requires maintainer review under the repository contribution policy.

[codecov]

Codecov Report

❌ Patch coverage is 68.27957% with 295 lines in your changes missing coverage. Please review.
✅ Project coverage is 74.03%. Comparing base (3282be2) to head (260bedc).
⚠️ Report is 6 commits behind head on main.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
cashu/payment/grpc/payment_processor_pb2.py	12.90%	81 Missing ⚠️
cashu/payment/grpc_processor.py	74.87%	49 Missing ⚠️
cashu/payment/grpc/payment_processor_pb2_grpc.py	59.74%	31 Missing ⚠️
cashu/mint/startup.py	21.62%	29 Missing ⚠️
cashu/payment/registry.py	62.00%	19 Missing ⚠️
cashu/wallet/crud.py	43.75%	18 Missing ⚠️
cashu/wallet/cli/cli.py	62.79%	16 Missing ⚠️
cashu/mint/router.py	78.43%	11 Missing ⚠️
cashu/payment/base.py	85.93%	9 Missing ⚠️
cashu/wallet/v1_api.py	86.44%	8 Missing ⚠️
... and 9 more

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1131      +/-   ##
==========================================
- Coverage   74.85%   74.03%   -0.82%     
==========================================
  Files         112      119       +7     
  Lines       12589    13492     +903     
==========================================
+ Hits         9423     9989     +566     
- Misses       3166     3503     +337     

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