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>

src/messaging/flow/flow-variable.service.ts

@@ -4,9 +4,15 @@ import { Injectable } from '@nestjs/common';
 export class FlowVariableService {
     // Replace {{variableName}} with values from session variables
     interpolate(template: string, variables: Record<string, any>): string {
-        return template.replace(/\{\{(\w+)\}\}/g, (match, name) => {
-            const value = variables[name];
-            if (value === undefined || value === null) return match; // keep placeholder if unresolved
+        return template.replace(/\{\{([\w.]+)\}\}/g, (match, path) => {
+            // Support dot notation: {{bookingResult.appointmentId}}
+            const parts = path.split('.');
+            let value: any = variables;
+            for (const part of parts) {
+                value = value?.[part];
+                if (value === undefined) return match;
+            }
+            if (value === null) return match;
             if (typeof value === 'object') return JSON.stringify(value);
             return String(value);
         });