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/types.ts

@@ -0,0 +1,27 @@
+export type NormalizedMessage = {
+    phone: string;           // E.164 without +, e.g. "919949879837"
+    name: string;            // sender name from WhatsApp profile
+    text: string;            // message text (or button reply title)
+    type: 'text' | 'interactive_reply' | 'location' | 'image' | 'unknown';
+    interactiveReply?: {     // populated when user taps a button or list item
+        id: string;          // button/row ID set by us
+        title: string;       // display text
+    };
+    rawPayload: any;         // original provider payload for debugging
+};
+
+export type ConversationEntry = {
+    role: 'user' | 'assistant';
+    content: string;
+    timestamp: number;
+};
+
+export type InteractiveButton = {
+    id: string;
+    title: string;           // max 20 chars for WhatsApp
+};
+
+export type ListSection = {
+    title: string;
+    rows: { id: string; title: string; description?: string }[];
+};