Admin Moderation Tools + Audit Trail (Referrals)

[Aditya8369]

Issue Details

Description: Admins can moderate referrals: flag spam, hide content, suspend poster, and see an audit log of moderation actions.

Proposed plan:
Backend: Add ReferralModerationAction model with adminId, referralId, actionType, reason, createdAt.
API: Admin-only endpoints:
POST /api/admin/referrals/:id/flag
POST /api/admin/referrals/:id/hide
POST /api/admin/referrals/:id/restore
Enforcement: Update referral queries (getReferrals) to exclude hidden/removed referrals for normal users.
Frontend: Add moderation section in existing admin referrals UI (there is frontend/src/admin/AdminReferrals.jsx in the repo).
Audit: Always log the action and reason.