feat: config-driven flow runtime engine for WhatsApp conversations

Groups + Blocks execution model adapted from Typebot:
- FlowExecutionService: walks through groups/blocks, pauses at InputBlocks
- FlowSessionService: Redis-backed session state (24h TTL)
- FlowStoreService: loads flow definitions from data/flows/ JSON files
- FlowVariableService: {{variable}} interpolation + expressions
- ToolRegistry: registered tool handlers (departments, doctors, slots, booking)
- Default appointment-booking.json flow seeded on first run

MessagingService delegates to flow engine when published flows exist,
falls back to hardcoded AI chat otherwise (backward compatible).

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

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

@@ -0,0 +1,44 @@
+import { Injectable } from '@nestjs/common';
+
+@Injectable()
+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
+            if (typeof value === 'object') return JSON.stringify(value);
+            return String(value);
+        });
+    }
+
+    // Interpolate all string values in an object
+    interpolateObject(obj: Record<string, string>, variables: Record<string, any>): Record<string, any> {
+        const result: Record<string, any> = {};
+        for (const [key, value] of Object.entries(obj)) {
+            result[key] = this.interpolate(value, variables);
+        }
+        return result;
+    }
+
+    // Execute expressions for SetVariableBlock
+    evaluateExpression(expression: string, value: string, variables: Record<string, any>): any {
+        switch (expression) {
+            case 'extract_id': {
+                // Extract UUID from "doc:{uuid}:{name}" or "dept:{name}" or "slot:{id}:{datetime}"
+                const parts = value.split(':');
+                return parts.length >= 2 ? parts[1] : value;
+            }
+            case 'date_tomorrow': {
+                const d = new Date(Date.now() + 86400000);
+                return d.toISOString().split('T')[0];
+            }
+            case 'date_day_after': {
+                const d = new Date(Date.now() + 2 * 86400000);
+                return d.toISOString().split('T')[0];
+            }
+            default:
+                return this.interpolate(value, variables);
+        }
+    }
+}