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

[angelTomo9]

Closes #2

Summary of Deliverables

Implements the referral reward loop engine in src/growth/referral.ts with SQL schema migrations and test coverage.

Features & Deliverables

• Database Schema: migrations/add_referral_system.sql creating referral_codes, referral_conversions, and system_events tables.
• Referral Engine: src/growth/referral.ts providing:
  • Code generation (createReferralCode)
  • Conversion processor (processReferral)
  • 5-credit award mechanism for referrers
  • Idempotency guard (prevents duplicate conversions for the same referred user)
  • Self-referral protection (blocks user from claiming own code)
  • Event logging to system_events with metadata.
• Test Suite: tests/referral.test.ts verifying happy path, credit allocation, duplicate rejection, and self-referral handling.