Feature/fraud detection

[soraiayugulis]

Summary

Implements a decoupled fraud detection module with a basic rule set and fail-safe integration into the authorization flow.

Changes

Interface

• FraudDetectionService — domain-level interface with isFraudulent(Card, amount, recentTransactions) method

Implementation

• BasicFraudDetectionService — detects fraud when 3+ recent transactions target the same merchant (rapid repeated transaction rule)

Integration

• AuthorizationService now accepts an optional FraudDetectionService
• Fraud check runs after daily limit check, before balance check
• Returns FRAUD_DETECTED rejection reason when triggered
• Fail-safe: if fraud detection throws an exception, the transaction proceeds (does not block legitimate transactions)

Tests

• 100 unit tests — all passing
• Covers: normal transactions pass, rapid repeated transactions detected, different merchants pass, decoupled architecture, fail-safe behavior

Design Decisions

• Fraud detection is injected as a dependency (decoupled from core authorization)
• Null-safe: when no fraud service is provided, authorization proceeds normally
• Fail-safe: exceptions in fraud detection do not block transactions