Feature: Referrals — add referral programme

[Hexstar-labs]

Description:
Users who refer new students should earn BST tokens.

Tasks:

• Add referralCode: string (unique, auto-generated) to User entity
• Add referredBy: string (nullable FK to User) to User entity
• On registration with a valid ?ref=CODE, set referredBy
• When the referred user completes their first course, mint 50 BST to the referrer via the Token contract
• Add GET /users/:id/referrals endpoint showing referral count and earned BST
• Add referral link sharing UI to the profile page

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