fix(settlement-engine): remove circular dependency with api-gateway

[mayborn005]

Summary

• Remove HTTP call from /api/settlements/reconcile endpoint to api-gateway, eliminating the circular dependency
• Replace cross-service comparison with local consistency validation that checks mathematical accuracy, fee calculations, and merchant references
• Remove the hand-rolled signHS256 JWT generation function that was only used for this HTTP call
• Add comprehensive test suite with 20 tests verifying reconciliation works without any HTTP calls

Changes

Removed

• signHS256() function for JWT generation
• HTTP fetch call to api-gateway/api/settlements
• Cross-service settlement record comparison logic

Added

• Local consistency check that validates:
  • Mathematical consistency: grossAmount - feeAmount = netAmount
  • Fee calculation accuracy: feeAmount matches feeBps applied to grossAmount
  • Merchant reference validity: all settlements reference existing merchants
• reconciliation.test.ts with 20 unit tests
• Status breakdown and summary statistics in reconciliation response

Updated Response Format

The reconcile endpoint now returns:

{
  "summary": { "total": 10, "valid": 9, "inconsistent": 1 },
  "statusBreakdown": { "pending": 2, "completed": 8 },
  "totals": { "gross": "1000", "fee": "10", "net": "990" },
  "inconsistencies": [...],
  "reconciliationType": "local_consistency_check"
}

Test plan

• All 63 settlement-engine tests pass (including 20 new reconciliation tests)
• Verify reconciliation works without api-gateway running
• Manual test: call /api/settlements/reconcile endpoint
• Verify no HTTP calls are made during reconciliation

Closes Betta-Pay#247