feat(pos): replace Meta Cloud API WhatsApp with Evolution API (self-hosted)

Switch from Meta Business Cloud API to Evolution API for WhatsApp integration.
Evolution API is self-hosted, free, and connects via WhatsApp Web QR code scan.

- Add docker-compose for Evolution API deployment
- Rewrite whatsapp_service.py for Evolution API endpoints
- Add instance management (create, QR, status, logout) to blueprint
- Add QR code scanning UI with connection status indicator
- Add duplicate message prevention in webhook handler
- Update config.py with EVOLUTION_API_URL/KEY (remove old Meta vars)
- Add setup documentation

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

pos/config.py

@@ -32,10 +32,9 @@ SMTP_USER = os.environ.get('SMTP_USER', '')
 SMTP_PASS = os.environ.get('SMTP_PASS', '')
 SMTP_FROM = os.environ.get('SMTP_FROM', 'noreply@nexusautoparts.com')
 
-# 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")
+# Evolution API (self-hosted WhatsApp via WhatsApp Web protocol)
+EVOLUTION_API_URL = os.environ.get('EVOLUTION_API_URL', 'http://localhost:8080')
+EVOLUTION_API_KEY = os.environ.get('EVOLUTION_API_KEY', 'nexus-evolution-key-2026')
 
 # Multi-currency
 DEFAULT_CURRENCY = os.environ.get('DEFAULT_CURRENCY', 'MXN')