feat(growth): implement referral reward loop with duplicate prevention (#2)

[angelTomo9]

Summary

Resolves #2 by implementing the Referral Reward Loop system for automatic credit disbursement.

Acceptance Criteria Checklist

• SQL Schema:
  eferral_codes and
  eferral_conversions tables in migrations/add_referral_system.sql.
• Logic Implementation: processReferral(code, newUserId, db) and generateReferralCode(ownerId, customCode, db) in src/growth/referral.ts.
• Credit Awarding: Automatically credits 5 free units to referrer on successful conversion.
• System Event Logging: Emits
  eferral_conversion event to audit log.
• Idempotency & Duplicate Prevention: Rejects duplicate redemption by same user and prevents self-referrals.
• Unit Tests: Full test suite covering happy path, duplicate prevention, and invalid codes in ests/referral.test.ts.

Wallet: 0x9f8F72aA9304c8B593d555F12eF6589cC3A579A2

Closes #2