Update email queue service documentation

Summary

This MR adds end-to-end documentation for the scalable email infrastructure feature in framework/dev/docs/EMAIL_QUEUE.md.

The documentation covers the complete email queue flow, architecture, database structure, job lifecycle, processing and retry/failure handling, rate limiting, branded email templates, and how the different components interact — plus a walkthrough for debugging a specific email.

Changes

• Added framework/dev/docs/EMAIL_QUEUE.md.
• Documented the end-to-end email queue workflow and architecture.
• Documented the email_jobs database structure and job lifecycle (including the pending → processing → sent/failed/cancelled flow and the retrying display label).
• Documented retry/backoff and watchdog handling.
• Documented Redis-backed rate limiting (RateLimiterService) on the forgot-password flow.
• Added a debugging guide (SQL/Redis/log commands to trace one email end to end).