Fix config assets and payment rate limiting

[Tomoya0k]

Summary

• Fix Horizon payment reconciliation to use accepted_assets instead of the removed usdc_issuer config field.
• Keep Config's manual Debug implementation so secrets remain redacted.
• Add RATE_LIMIT_REQUESTS_PER_SEC config support and wire rate limiting for POST /payments.
• Use Axum ConnectInfo for real client IPs, with x-forwarded-for / x-real-ip as fallback.
• Add API coverage for 429 rate_limit_exceeded.

Validation

• cargo check
• cargo test

Notes

Cargo.lock was updated to include governor and its transitive dependencies, matching the existing Cargo.toml dependency.

Merge Update

• Merged upstream/main into this branch.
• Kept the upstream merchant auth, API keys, shutdown coordination, migrations, and config validation changes.
• Preserved the branch-specific rate limiting refinement:
  • RateLimitState scoped to the router
  • POST /payments only
  • ConnectInfo IP first, then x-forwarded-for / x-real-ip fallback
• Re-ran the full test suite after merge.

[GBOYEE]

I'd like to work on this.

Approach:

• I'll add the CI config and test it locally before pushing
• Verify with existing tests + add new ones if needed

Estimated effort: ~1-2 hours. PR incoming shortly.