feat(pos): WhatsApp Business API integration — send/receive messages, quotations

Add full WhatsApp Cloud API integration for Nexus POS:
- Service layer (whatsapp_service.py): send text, templates, quotations,
  order confirmations, stock alerts; process incoming webhooks with AI auto-reply
- Blueprint (whatsapp_bp.py): public webhook endpoints for Meta verification +
  incoming messages; authenticated endpoints for send, send-quote, conversations
- Conversation UI (whatsapp.html + whatsapp.js): split-panel messenger with
  conversation list, chat bubbles, send input, quote sending; both themes
- Migration v1.4: whatsapp_messages table with phone/direction/status indexes
- Config: WHATSAPP_TOKEN, WHATSAPP_PHONE_ID, WHATSAPP_VERIFY_TOKEN env vars
- Sidebar: WhatsApp nav item under Gestion with message-bubble icon
- Ready for Meta Business credentials (infrastructure complete, no API keys needed)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

pos/config.py

@@ -24,3 +24,8 @@ OPENROUTER_API_KEY = os.environ.get(
     "OPENROUTER_API_KEY",
     "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"
 )
+
+# WhatsApp Business Cloud API
+WHATSAPP_TOKEN = os.environ.get("WHATSAPP_TOKEN", "")
+WHATSAPP_PHONE_ID = os.environ.get("WHATSAPP_PHONE_ID", "")
+WHATSAPP_VERIFY_TOKEN = os.environ.get("WHATSAPP_VERIFY_TOKEN", "nexus-wa-verify-2026")