feat(fraud): implement AI-assisted fraud detection for campaign listings

[devogechukwu]

Summary of Changes

Implements AI-assisted fraud detection for campaign listings before they go live, satisfying all acceptance criteria:

1. Automated Review on Campaign Publish:

   • Evaluates campaign metadata, duration, tree targets, and textual risk indicators upon submission (POST /api/campaigns).
   • Restricts campaign listing activation (PATCH /api/campaigns/[id]) if flagged until manual admin review is completed.

2. Manual Review Queue:

   • Added in-memory queue management for flagged listings with risk scores and evidence flags.
   • Added endpoints:
     • GET /api/admin/fraud/review-queue: view and filter pending/reviewed items.
     • POST /api/admin/fraud/review-queue/[id]/decision: approve or reject campaigns with admin audit logging.

3. False Positive Rate Monitoring & Quarterly Model Retraining:

   • Tracks evaluations, flagged count, approved count (false positives), rejected count (true positives), and calculates falsePositiveRate.
   • Implements 90-day quarterly retraining interval checks (isQuarterlyRetrainingDue) and versioned retraining logs (retrainModel).
   • Added endpoints:
     • GET /api/admin/fraud/metrics: view live model metrics and retraining due status.
     • POST /api/admin/fraud/retrain: trigger scheduled or manual model retraining.

4. Tests:

   • Unit tests for automated listing review, manual review queue, false positive rate calculation, and quarterly retraining.
   • API route tests for campaign publishing and admin fraud review endpoints.

Closes #1263

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