Fix settlement cancel reason length

[d3v-active]

Closes #159

Added a 500-character maximum length validation on the optional reason field in SettlementService.cancel.

Motivation and Context
This prevents potential storage-amplification and log-noise issues caused by unbounded client-supplied text, while keeping the existing optional, non-blank behavior unchanged.

Changes

• Added requireStringMaxLength to src/utils/validation.ts.
• Applied requireStringMaxLength to SettlementService.cancel in src/services/settlementService.ts to enforce a 500-character limit.
• Added unit tests in src/services/settlementService.test.ts.
• Added integration tests in src/routes/settlements.test.ts.