fix: add fallback for databases without transaction support

Some database adapters don't support transactions, causing payment
updates to fail completely. This change adds graceful fallback to
direct updates when transactions are unavailable.

Changes:
- Try to use transactions if supported
- Fall back to direct update if beginTransaction() fails or returns null
- Add debug logging to track which path is used
- Maintain backward compatibility with transaction-supporting databases

This fixes the "Failed to begin transaction" error in production.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

package.json

@@ -1,6 +1,6 @@
 {
   "name": "@xtr-dev/payload-billing",
-  "version": "0.1.27",
+  "version": "0.1.28",
   "description": "PayloadCMS plugin for billing and payment provider integrations with tracking and local testing",
   "license": "MIT",
   "type": "module",