feat: AI chat system with LangGraph, multi-step patient flows, and appointment rescheduling

- Add floating chat window Hand-off to AI toggle (per-patient) and after-hours AI toggle (global)
- Add LangGraph-powered appointment reminder flow: AI introduces itself, classifies YES/NO, handles confirmation with appointment date/time
- Add multi-step rescheduling flow: ASAP vs next week, tomorrow offer, Mon/Tue/Wed picker, morning/afternoon time slot — automatically updates appointment in DB
- Add new patient / after-hours flow: new vs existing patient, dental insurance check, MassHealth Selenium eligibility check (auto-uses saved member ID + DOB for existing patients), self-pay fallback
- Add AI Chat Settings page (Settings → Advanced) with editable greeting templates and LangGraph flow diagrams for both reminder and new-patient flows
- Add Schedule a New Patient template option in chat window, starts new-patient conversation flow
- Add GET/PUT endpoints for AI handoff, after-hours handoff, and AI chat templates
- Add multilingual support (7 languages) across all AI reply nodes with LLM generation and hardcoded fallbacks
- Add pending reschedule in-memory store and conversation stage tracking across all flows

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

apps/Backend/src/storage/twilio-storage.ts

@@ -59,6 +59,30 @@ export const twilioStorage = {
     });
   },
 
+  async getAiChatTemplates(userId: number): Promise<Record<string, string>> {
+    const settings = await db.twilioSettings.findUnique({ where: { userId } });
+    const all = (settings?.templates as Record<string, string>) || {};
+    return {
+      reminderGreeting:    all["_ai_chat_reminder_greeting"]    ?? "",
+      newPatientGreeting:  all["_ai_chat_new_patient_greeting"] ?? "",
+      generalFallback:     all["_ai_chat_general_fallback"]     ?? "",
+    };
+  },
+
+  async saveAiChatTemplates(userId: number, templates: { reminderGreeting?: string; newPatientGreeting?: string; generalFallback?: string }) {
+    const settings = await db.twilioSettings.findUnique({ where: { userId } });
+    const existing = (settings?.templates as Record<string, string>) || {};
+    const updated: Record<string, string> = { ...existing };
+    if (templates.reminderGreeting   !== undefined) updated["_ai_chat_reminder_greeting"]    = templates.reminderGreeting;
+    if (templates.newPatientGreeting !== undefined) updated["_ai_chat_new_patient_greeting"]  = templates.newPatientGreeting;
+    if (templates.generalFallback    !== undefined) updated["_ai_chat_general_fallback"]      = templates.generalFallback;
+    return db.twilioSettings.upsert({
+      where:  { userId },
+      update: { templates: updated },
+      create: { userId, accountSid: "", authToken: "", phoneNumber: "", templates: updated },
+    });
+  },
+
   async getRecentCommunicationsByUser(userId: number, limit = 20) {
     return db.communication.findMany({
       where: { patient: { userId } },