feat: appointment QR code — generated and sent via WhatsApp after booking

- QrService: generates QR PNG from appointment data, cached in-memory
- GET /api/messaging/qr/:appointmentId serves the image (Gupshup needs URL)
- sendImage added to MessagingProvider + GupshupProvider
- send_appointment_qr tool registered in ToolRegistry
- Flow JSON updated: QR sent after booking confirmation
- Variable interpolation now supports dot notation ({{result.field}})
- SIDECAR_PUBLIC_URL env var for the QR image URL

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

package.json

@@ -32,11 +32,13 @@
     "@nestjs/platform-express": "^11.0.1",
     "@nestjs/platform-socket.io": "^11.1.17",
     "@nestjs/websockets": "^11.1.17",
+    "@types/qrcode": "^1.5.6",
     "ai": "^6.0.116",
     "axios": "^1.13.6",
     "ioredis": "^5.10.1",
     "json-rules-engine": "^6.6.0",
     "kafkajs": "^2.2.4",
+    "qrcode": "^1.5.4",
     "reflect-metadata": "^0.2.2",
     "rxjs": "^7.8.1",
     "socket.io": "^4.8.3",