Add `?minAmount=`/`?maxAmount=` range filtering to `GET /api/v1/settlements`

[Jagadeeshftw]

📌 Description

routes/settlements.ts's GET / supports filtering by ?anchor= and ?asset= only. An operator investigating settlements above/below a certain size (e.g. auditing unusually large settlements) currently has to fetch everything and filter client-side.

🧩 Requirements and context

• Accept optional ?minAmount=/?maxAmount= query params, validated as non-negative numbers via utils/validation.ts.
• Apply the range filter in SettlementService.list (extend its filters parameter) rather than in the route handler, consistent with the existing anchor/asset filtering pattern.
• Combine correctly with existing ?anchor=/?asset= filters, sorting, and pagination.

🛠️ Suggested execution

• Extend src/services/settlementService.ts's list(filters) signature to accept minAmount/maxAmount and filter accordingly.
• Modify src/routes/settlements.ts's GET / handler to parse and pass the new query params through.
• Update src/openapi.ts's /api/v1/settlements GET parameters.
• Add tests in src/services/settlementService.test.ts and src/routes/settlements.test.ts.

✅ Acceptance criteria

• GET /api/v1/settlements?minAmount=100&maxAmount=500 returns only settlements within that range.
• Range filters combine correctly with ?anchor=, ?asset=, sorting, and pagination.
• Invalid (non-numeric or negative) range values return 400.

🔒 Security notes

N/A — read-path filter over already-exposed settlement data.

📋 Guidelines

• Minimum 95% test coverage
• Clear documentation
• Timeframe: 96 hours