docs: add animated SVG architecture diagram

[ojusave]

Summary

• Adds docs/architecture.html — a self-contained, animated architecture diagram for Arlo
• Single file, inline SVG, inline vendor logos (no external assets, no diagram library)
• Animated dashed flow arrows preview the live RTMS transcript stream; intended to be screen-recorded to GIF for the README

What it shows

Top-down flow:

1. Meeting Participant → Zoom Meeting with embedded Arlo Zoom App
2. Arlo Backend panel containing seven numbered stages:
   • oauth_callback — Zoom OAuth 2.0 (PKCE), AES-256-GCM token storage
   • rtms_webhook — meeting.rtms_started / _stopped events with signature validation
   • rtms_stream — long-lived WebSocket to Zoom RTMS (<1s latency)
   • transcript_pipeline — normalise, buffer, batch-insert to Postgres, broadcast deltas
   • voice_command — "Arlo, …" wake-word router
   • ai_insights — OpenRouter (Gemini / Llama free tier) for summaries, action items
   • search_and_chat — Postgres FTS + RAG chat
3. Retry loop on LLM failure (routed outside the panel)
4. Postgres cylinder + optional Redis cylinder for WebSocket scaling

Test plan

• Open docs/architecture.html in a browser and confirm fonts load + dashes animate
• Screen-record the rendered diagram to docs/images/architecture-diagram.gif
• Reference the GIF in README.md under an ## Architecture heading