feat(whatsapp): QWEN primary AI backend, Hermes fallback, conversation history, vehicle persistence, demo prompts

- Add QWEN (qwen3.6) as primary AI backend with short system prompt
- Hermes remains as fallback with 45s timeout
- Increase QWEN timeout to 35s, max_tokens to 4000
- Add conversation history loading from whatsapp_messages (last 4 msgs)
- Persist detected vehicle in whatsapp_sessions table
- Add 'limpiar chat' / 'nuevo chat' / 'reset' commands to clear history
- Fix CSS conflict: rename whatsapp chat-panel classes to wa-chat-panel
- Fix JS ID conflicts with chat.js widget (waChatPanel, waChatMessages, etc.)
- Improve no-stock response: conversational with alternatives
- Split search_query by | for multi-part lookups
- Add DEMO_PROMPTS.md and DEMO_PROMPTS_V2.md

systemd/nexus-pos.service

@@ -18,6 +18,9 @@ Environment=REDIS_URL=redis://localhost:6379/0
 Environment=REDIS_ENABLED=true
 Environment=MEILI_URL=http://localhost:7700
 Environment=MEILI_ENABLED=true
+Environment=QWEN_API_URL=https://api.nan.builders/v1
+Environment=QWEN_API_KEY=sk-Yr0e-Y4F4j1NuuK8xdbxIA
+Environment=QWEN_MODEL=qwen3.6
 
 [Install]
 WantedBy=multi-user.target