Add API key authentication support for /api/chat (Bearer or X-API-Key header)

Simplify to single-model chat app: llama3.1:8b only. Add /api/chat SSE streaming, chatbot UI, remove model selection/download UI, build-time model pull

Replace simulated download with real ollama pull subprocess, add Ollama status check, installed models endpoint, WS wss support

Initial commit: FastAPI app with auth, API keys, download tracking, WebSocket live progress