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

Objective

Modify the sign-up flow to capture referral code from the /join/[CODE] URL, validate it, and persist attribution to the new user record.

Acceptance Criteria

• Referral code extracted from URL param or cookie (survives redirect to sign-up form)
• Code validated against referral_codes table (reject invalid/revoked codes)
• referred_by set on new user record upon successful sign-up
• Fires referee_signup_attributed analytics event
• Self-referral blocked (user can't use own code)

Dependencies

Depends on: referred_by migration + referral_codes table. Blocks: reward grant service.