feat: referral

[hyoban]

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• Feature
• Bugfix
• Hotfix
• Other (please describe):

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• I have updated the changelog/next.md with my changes.

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[follow-reviewer-bot]

Suggested PR Title:

feat: add referrals table and referral code support

Change Summary:
Introduced a new 'referrals' table, including its schema and API support, enabling referral code tracking and association with users. Updated API output types to expose referral data. Extended the auth client to attach 'folo-referral-code' from URL query params to request headers, allowing the backend to identify and process user referrals. This enhances the platform's capability to support referral programs and track invited users.

Code Review:
No change requests necessary.