docs(fraud): add cross-service fraud detection system guide

[Nanfe01]

docs(fraud): add cross-service fraud detection system guide

• document full fraud pipeline from backend to ML inference
• define scoring thresholds and decision logic
• describe alert creation and storage flow
• document failure modes and fallback behavior
• add feedback loop and retraining pipeline
• align backend and ml-service responsibilities
• update API.md, DEPLOYMENT.md, and ml-service README

closes #539

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ℹ️ Review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: 513695d2-ea7c-4b23-b24b-02b64157d3a3

📥 Commits

Reviewing files that changed from the base of the PR and between b356a85 and 7f21297.

📒 Files selected for processing (3)

• frontend/docs/FRAUD_DETECTION.md
• frontend/docs/SEARCH.md
• frontend/src/components/usePaymentCheckout.ts

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