feat: WhatsApp AI assistant — provider-agnostic messaging with Gupshup

Provider-agnostic WhatsApp integration for AI-driven appointment booking:

- MessagingProvider interface (sendText, sendButtons, sendList, parseInbound)
- GupshupProvider implementation (Gupshup WhatsApp API)
- MessagingService — AI orchestration with tools (department/doctor/slot
  lists via interactive WhatsApp messages, appointment booking, caller
  resolution + context injection)
- Redis conversation history (24h TTL, matches WhatsApp session window)
- Webhook controller at POST /api/messaging/webhook

Swappable to Ozonetel or Meta Cloud API by implementing MessagingProvider
and switching MESSAGING_PROVIDER env var.

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

src/messaging/messaging.controller.ts

@@ -0,0 +1,36 @@
+import { Controller, Post, Body, Logger } from '@nestjs/common';
+import { MessagingProvider } from './providers/messaging-provider.interface';
+import { MessagingService } from './messaging.service';
+
+@Controller('api/messaging')
+export class MessagingController {
+    private readonly logger = new Logger(MessagingController.name);
+
+    constructor(
+        private readonly provider: MessagingProvider,
+        private readonly messaging: MessagingService,
+    ) {}
+
+    @Post('webhook')
+    async webhook(@Body() body: any) {
+        this.logger.log(`[WA-WEBHOOK] Received: ${JSON.stringify(body).substring(0, 300)}`);
+
+        if (!this.provider.validateWebhook(body)) {
+            this.logger.warn('[WA-WEBHOOK] Validation failed — ignoring');
+            return { status: 'ignored', reason: 'validation failed' };
+        }
+
+        const message = this.provider.parseInbound(body);
+        if (!message) {
+            this.logger.log('[WA-WEBHOOK] Non-message event — skipped');
+            return { status: 'ok', type: body?.type ?? 'unknown' };
+        }
+
+        // Handle async — don't block webhook response
+        this.messaging.handleInbound(message).catch(err => {
+            this.logger.error(`[WA-WEBHOOK] handleInbound failed: ${err.message}`);
+        });
+
+        return { status: 'ok' };
+    }
+}