feat(growth): implement referral reward loop engine (#2)

[jihadMo]

Closes #2

Summary of Changes:

• Migration: Created migrations/add_referral_system.sql with idempotent referral_codes and referral_conversions tables and process_referral stored procedure.
• Engine Implementation: Implemented src/growth/referral.ts with ReferralEngine class providing createCode and processReferral methods, awarding 5 credits to referrers upon conversion and emitting referral_conversion system events.
• Idempotency: Enforced duplicate prevention key ${code}:${newUserId}.
• Tests: Added comprehensive test suite in tests/referral.test.ts covering happy path conversion, duplicate prevention, and invalid code rejection.

Wallet for Base USDC settlement:

0x3AD489c09A5C750b29155B0d0426Fc70654D59e5