feat: add referral rewards

[danielwpz]

Summary:

• Add referral codes, referral binding, and referral dashboard endpoint
• Grant invitee/inviter credits through existing credit tables when each referral's snapshotted trigger is met
• Add admin referral config and auth/OAuth referral-code handling

Validation:

• cargo fmt --all -- --check
• cargo clippy --all-targets --features test -- -D warnings

[gemini-code-assist]

Code Review

This pull request introduces a comprehensive referral system, including database migrations, a new ReferralService, and a ReferralRepository. It integrates referral tracking into the authentication flows (OAuth, email, NEAR) and the subscription lifecycle to grant credit rewards based on configurable triggers like registration or first active subscription. Additionally, it adds administrative configuration for referrals and a new API endpoint for user referral dashboards. I have no feedback to provide as no review comments were present.

[chatgpt-codex-connector]

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