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/messaging.service.ts

@@ -1,9 +1,10 @@
-import { Injectable, Logger } from '@nestjs/common';
+import { Injectable, Inject, Logger, Optional } from '@nestjs/common';
 import { ConfigService } from '@nestjs/config';
 import { generateText, tool, stepCountIs } from 'ai';
 import { z } from 'zod';
 import { MessagingProvider } from './providers/messaging-provider.interface';
 import { MessagingConversationService } from './messaging-conversation.service';
+import { FlowExecutionService } from './flow/flow-execution.service';
 import { CallerResolutionService } from '../caller/caller-resolution.service';
 import { CallerContextService } from '../caller/caller-context.service';
 import { PlatformGraphqlService } from '../platform/platform-graphql.service';
@@ -27,6 +28,7 @@ export class MessagingService {
         private callerContext: CallerContextService,
         private platform: PlatformGraphqlService,
         private aiConfig: AiConfigService,
+        @Optional() private flowExecution: FlowExecutionService,
     ) {
         const cfg = aiConfig.getConfig();
         this.aiModel = createAiModel({
@@ -51,6 +53,14 @@ export class MessagingService {
         const replyId = message.interactiveReply?.id;
         this.logger.log(`[WA] Inbound from ${phone} (${name}): ${text.substring(0, 100)}${replyId ? ` [reply_id=${replyId}]` : ''}`);
 
+        // Delegate to flow engine if published flows exist
+        if (this.flowExecution?.hasFlows()) {
+            this.logger.log(`[WA] Delegating to flow engine`);
+            await this.flowExecution.handleMessage(message);
+            return;
+        }
+
+        // Fallback: hardcoded AI chat (legacy — will be removed once flows are validated)
         if (!this.aiModel) {
             await this.provider.sendText(phone, 'Our assistant is temporarily unavailable. Please call us directly.');
             return;