US-014 - Configurable fraud thresholds per account tier

[Teboho66]

As a Fraud Analyst, I want configurable fraud detection rule thresholds per account tier so that the system balances fraud catch rate against false positive rate differently for Standard, Premium, and Business customers.

Acceptance Criteria:

• Threshold changes applied via Admin API within 60 seconds without service restart
• Decision output includes threshold_applied and account_tier fields
• All rule changes logged in audit trail with timestamp and actor ID
• Separate thresholds configurable for Standard, Premium, and Business tiers

Traceability: FR-06, FR-07, UC4
Story Points: 3
Priority: Low | MoSCoW: Won't-have (Sprint 1)