[2026-04-30][refactor][general] replace string-based association lookups with explicit model references across billing and subscription services

👤 Author : Matoraaa
🌿 Branch : master
🔗 Commit : 55babd5
📅 Date : 2026-04-30
📝 Note :
refactor: replace string-based association lookups with explicit model references across billing and subscription services