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/providers/messaging-provider.interface.ts

@@ -0,0 +1,18 @@
+import { NormalizedMessage, InteractiveButton, ListSection } from '../types';
+
+export abstract class MessagingProvider {
+    /** Parse raw webhook payload into normalized message */
+    abstract parseInbound(body: any): NormalizedMessage | null;
+
+    /** Send a plain text message */
+    abstract sendText(to: string, text: string): Promise<void>;
+
+    /** Send interactive buttons (max 3 for WhatsApp) */
+    abstract sendButtons(to: string, body: string, buttons: InteractiveButton[]): Promise<void>;
+
+    /** Send interactive list (max 10 rows total across sections) */
+    abstract sendList(to: string, body: string, buttonText: string, sections: ListSection[]): Promise<void>;
+
+    /** Validate that inbound webhook is authentic */
+    abstract validateWebhook(body: any): boolean;
+}