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

pos/templates/inventory.html

@@ -815,7 +815,7 @@
   <script src="/pos/static/js/pos-utils.js" defer></script>
   <script src="/pos/static/js/sidebar.js" defer></script>
   <script src="/pos/static/js/virtual-scroll.js" defer></script>
-  <script src="/pos/static/js/inventory.js?v=2" defer></script>
+  <script src="/pos/static/js/inventory.js?v=3" defer></script>
   <script src="/pos/static/js/offline-banner.js" defer></script>
   <script src="/pos/static/js/sync-engine.js" defer></script>
   <script>if('serviceWorker' in navigator){navigator.serviceWorker.register('/pos/sw.js',{scope:'/pos/'});}</script>