docs: add comprehensive review of subscription and marketplace functionality

Commit notes

- Identify 3 critical issues: OAuth state in-memory, missing transaction in purchase completion, missing redirect URL validation - Document 2 medium issues: silent webhook error handling, payout race condition - List 3 minor issues: console.log usage, confusing storage logic, missing index - Include positive observations and actionable recommendations - Prioritized action items for remediation

Co-authored-by: armin.naimi <[email redacted]>