docs: document Aurora Serverless v2 and demo environment behaviors

Updated architecture documentation to reflect actual infrastructure:
- Changed from traditional RDS to Aurora Serverless v2 (0.5-16 ACU)
- Added "Known Behaviors" section explaining:
  • Aurora cold start delay (5-10s after idle)
  • HTTP→HTTPS redirect delay due to missing port 80 listener
- Updated ASCII diagram and storage layer details
- Added load time expectations table for demos

These behaviors are acceptable for demo environments where cost
optimization (~$120/month savings) outweighs instant response time.

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

Co-Authored-By: Claude <[email protected]>

Author: Noah Boyers