Phase 1 | Priority: P0 | Status: Not Started | Covers: FR-003

Objective

Add a referred_by column to the users table to persist which referral code was used at sign-up, enabling attribution tracking.

Acceptance Criteria

• New nullable column: referred_by (FK to referral_codes.id)
• Index on referred_by for reporting queries
• Migration runs cleanly; no impact on existing users

Dependencies

Depends on: referral_codes table. Blocks: attribution tracking in sign-up flow.