Add comprehensive architecture documentation (#177)

* Add comprehensive architecture documentation

Created Architecture.md with detailed documentation covering:
- System overview with Mermaid diagrams
- Backend architecture including RAG pipeline
- Frontend React/TypeScript structure
- Deployment infrastructure on Digital Ocean
- Secrets management and security measures

Addresses requirements from issue #176.

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

Co-authored-by: yangm2 <yangm2@users.noreply.github.com>

* Add detailed technical sections to Architecture.md

- Add comprehensive RAG implementation details including type/category (tool-augmented RAG with Vertex AI)
- Document multi-turn conversation management with session architecture and persistence details
- Add complete streaming response implementation covering both backend and frontend
- Include code examples, Mermaid diagrams, and technical specifications
- Cover session data structures, conversation flow, and performance benefits

Co-authored-by: yangm2 <yangm2@users.noreply.github.com>

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Co-authored-by: claude[bot] <209825114+claude[bot]@users.noreply.github.com>

Author: yangm2