docs(queue/sql): add comprehensive RFC for SQL queue design

## Why?
Document the design decisions, alternatives, and rationale behind the SQL queue implementation for future reference and onboarding.

## What?
- Comprehensive RFC covering requirements, alternatives, and trade-offs
- Detailed explanation of partition leasing and visibility timeout mechanisms
- Performance characteristics and scalability considerations
- Migration plan and future work roadmap
- Example queries and configuration reference

## Test Plan
- RFC reviewed for technical accuracy
- All described mechanisms match implementation
- Examples verified against actual code

Author: behinddwalls