Adds nullable FK campaign_id to referral_earnings table, enabling
direct campaign ROI analytics without JOINing through registrations.
- Model: campaign_id column + AdvertisingCampaign relationship
- CRUD: get_user_campaign_id() helper, campaign_id param in create_referral_earning
- Service: resolve campaign_id in all earning creation paths
- Cabinet API: campaign_name in earnings response
- Migration 0002: add column + deterministic backfill via DISTINCT ON