[Backend] -- Add Referral Fraud Detection Heuristics

[Devsol-01]

Description:
Referral programs can be abused with fake signups or self-referrals. Implement heuristic checks:

• detect self-referral patterns
• detect repeated referral attempts with similar metadata
• rate-limit referral creation
• quarantine suspicious referrals for manual review

Acceptance Criteria:

• Fraud detection rules implemented.
• Suspicious referrals flagged.
• Audit trail for suspicious actions.

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