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>

docker/docker-compose.evolution.yml

@@ -0,0 +1,23 @@
+version: '3'
+services:
+  evolution-api:
+    image: atendai/evolution-api:latest
+    container_name: evolution-api
+    restart: always
+    ports:
+      - "8080:8080"
+    environment:
+      - SERVER_URL=http://localhost:8080
+      - AUTHENTICATION_API_KEY=nexus-evolution-key-2026
+      - DATABASE_ENABLED=true
+      - DATABASE_PROVIDER=postgresql
+      - DATABASE_CONNECTION_URI=postgresql://nexus:nexus_autoparts_2026@host.docker.internal:5432/evolution_api
+      - DATABASE_CONNECTION_CLIENT_NAME=evolution
+      - QRCODE_LIMIT=10
+      - WEBHOOK_GLOBAL_URL=http://host.docker.internal:5001/pos/api/whatsapp/webhook
+      - WEBHOOK_GLOBAL_ENABLED=true
+      - WEBHOOK_EVENTS_MESSAGES_UPSERT=true
+    volumes:
+      - evolution_data:/evolution/instances
+volumes:
+  evolution_data: