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

[mayborn005]

Remove HTTP call from /api/settlements/reconcile endpoint to api-gateway. The reconciliation now performs a local consistency check against the settlement engine's own database instead of calling the api-gateway.

Changes:

• Remove signHS256 JWT generation function
• Remove HTTP fetch to api-gateway settlement endpoint
• Replace cross-service comparison with local consistency validation:
  • Mathematical consistency: grossAmount - feeAmount = netAmount
  • Fee calculation accuracy: feeAmount matches feeBps
  • Merchant reference validity: settlements reference existing merchants
• Add comprehensive test suite for reconciliation logic (20 tests)
• Update test script to include reconciliation tests

Closes #247

Description

Please include a clear, concise summary of the changes introduced by this pull request. Explain the rationale behind the solution, any design trade-offs made, and context on how the changes achieve the goal.

Fixes / Closes: # (issue number)

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Type of Change

• Bug Fix (non-breaking change which fixes an issue)
• New Feature (non-breaking change which adds functionality)
• Breaking Change (fix or feature that would cause existing functionality to not work as expected)
• Refactoring / Performance (clean-up or performance optimization without behavioral changes)
• Documentation / CI (changes to docs, workflows, config files)

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Verification & Test Plan

Please outline how you verified these changes. Include steps to reproduce, test scripts, or command logs where applicable.

Automated Tests

For example: cargo test in contract package, pnpm test in backend.

# Provide the commands run to test the code

Manual Verification

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Sub-System Checklist

1. Smart Contracts (BettaPay-Contract)

• Code compiles without warnings (cargo build --release --target wasm32-unknown-unknown)
• Cargo test suite passes locally (cargo test)
• Emits events for state changes
• Implemented proper authorization controls (auth.require_auth())

2. Backend Services (BettaPay-Backend)

• Prisma models updated and migrated (if database schema changes)
• Zod validation checks added/updated
• API routes and services build successfully (pnpm build)
• Local env variables updated in .env.example

3. Frontend Dashboard (BettaPay-Frontend)

• Local build completes successfully (pnpm build)
• Focus states, labels, and aria attributes tested for accessibility
• No regression on styling or components
• Env parameters documented in .env.local

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Checklist

• My code follows the style guidelines of this project (run formatters/linters)
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings