fix: address critical issues in subscription and marketplace functionality

Commit notes

Critical fixes: 1. OAuth state storage: Move from in-memory Map to Redis for production reliability and horizontal scaling support (mollie-connect.ts)

2. Purchase completion: Wrap status update and earnings credit in a database transaction to ensure atomicity (purchases.ts)

3. Redirect URL validation: Add isValidRedirectUrl() check to subscription checkout and resume endpoints to prevent open redirect attacks (subscription.ts)

Medium fixes: 4. Webhook logging: Improve error logging from debug to warn level when no handler processes a webhook, with detailed error information (subscription.ts)

5. Payout race condition: Use transaction for atomic balance update and payout creation, with accurate error messages on failure (purchases.ts)

Minor fixes: 6. Structured logging: Replace console.log/error with structured logger in sample-packs.ts for consistent log formatting

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