Referral system refresh

Reworked the referral system end to end:

Anti-fraud cooldowns configurable from 0–90 days between referrer and referee orders
Per-customer caps on referral bonuses per month and per lifetime
Milestone bonuses for every 5th, 10th, 25th completed referral
Pending vs completed ledger views so merchants can audit in-flight referrals before they convert

Replaces the lightweight version that shipped at 1.0 launch.