Multi-model AI chat workspace

Build a team-friendly AI chat app.

Core features:
1. Chat with GPT / Claude / Gemini side-by-side (pick model per message)
2. Workspaces with shared prompt library
3. File upload → context for the conversation
4. Streaming replies, per-user usage/cost tracking
5. RLS so users only see their own or shared threads

Data: workspaces, threads, messages, files, usage.
Must-haves: markdown + code rendering, keyboard shortcuts, export thread.