Add request archive data lake design doc

[alekhya1098]

Summary

This PR adds a design document for archiving closed requests data from the operational database into an S3-based data lake.

The design covers:

• Problem statement and goals
• Functional and non-functional requirements
• Proposed AWS architecture
• Extraction strategy and tradeoffs
• S3 layout, partitioning, Parquet format, and lifecycle policy
• Glue Data Catalog and Athena query layer
• Schema evolution strategy
• PII and privacy handling
• Alternatives considered
• Failure modes and observability
• Cost estimate at current and 10x scale
• Rollout and rollback plan
• Open questions for the team

Notes

This is a design-doc-only change. No pipeline implementation is included.

Issue

References #175