Replace architecture Mermaid diagrams with SVG

[neuromechanist]

Summary

• Replace all Mermaid diagrams in architecture page with two SVG diagrams (overview + detailed)
• Split architecture into two pages: Overview and Details
• Correct inaccurate components that were never implemented (router agent, PostgreSQL, Redis, vector store, multiple instances, background workers)
• All diagrams now reflect the actual deployed system

Changes

• architecture.md: Rewritten as overview page with high-level SVG and accurate component descriptions
• architecture-details.md: New page with detailed infrastructure SVG, request flow, community routing, model selection, knowledge system, security, and CI/CD
• architecture-overview.svg: High-level diagram (researchers -> clients -> edge -> platform -> services)
• architecture-diagram.svg: Detailed infrastructure diagram with orthogonal arrow routing
• mkdocs.yml: Architecture nav expanded to Overview + Details

What was corrected

• Removed Router Agent / Supervisor pattern (not implemented)
• Removed PostgreSQL, Redis, Vector Store (actual: SQLite + FTS5, in-memory sessions)
• Removed multiple FastAPI instances / load balancer (actual: single instance)
• Removed background workers (actual: CLI-driven knowledge sync)
• Added actual Apache reverse proxy setup
• Added accurate BYOK authentication flow
• Added CI/CD pipeline details

Test plan

• mkdocs build succeeds with no errors
• Verify SVGs render correctly on the docs site
• Check dark mode appearance