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/routes/twilio.ts

@@ -1,6 +1,7 @@
 import express, { Request, Response } from "express";
 import twilio from "twilio";
 import { storage } from "../storage";
+import { getHandoff, setHandoff, resetConversation, startNewPatientConversation, getAfterHoursHandoff, setAfterHoursHandoff } from "../ai/aiHandoffStore";
 
 const router = express.Router();
 
@@ -65,7 +66,7 @@ router.post("/send-sms", async (req: Request, res: Response): Promise<any> => {
     const userId = req.user?.id;
     if (!userId) return res.status(401).json({ message: "Unauthorized" });
 
-    const { to, message, patientId } = req.body;
+    const { to, message, patientId, startFlow } = req.body;
     if (!to || !message) return res.status(400).json({ message: "to and message are required" });
 
     const settings = await storage.getTwilioSettings(userId);
@@ -81,8 +82,9 @@ router.post("/send-sms", async (req: Request, res: Response): Promise<any> => {
     });
 
     if (patientId) {
+      const pid = Number(patientId);
       await storage.createCommunication({
-        patientId: Number(patientId),
+        patientId: pid,
         userId,
         channel: "sms",
         direction: "outbound",
@@ -90,6 +92,12 @@ router.post("/send-sms", async (req: Request, res: Response): Promise<any> => {
         body: message,
         twilioSid: twilioMsg.sid,
       });
+      // Set conversation stage based on which flow was started
+      if (startFlow === "new_patient") {
+        startNewPatientConversation(userId, pid);
+      } else {
+        resetConversation(userId, pid);
+      }
     }
 
     return res.status(200).json({ sid: twilioMsg.sid, status: twilioMsg.status });
@@ -125,6 +133,60 @@ router.put("/templates/:key", async (req: Request, res: Response): Promise<any>
   }
 });
 
+// GET /api/twilio/after-hours-handoff
+router.get("/after-hours-handoff", async (req: Request, res: Response): Promise<any> => {
+  try {
+    const userId = req.user?.id;
+    if (!userId) return res.status(401).json({ message: "Unauthorized" });
+    return res.status(200).json({ enabled: getAfterHoursHandoff(userId) });
+  } catch (err) {
+    return res.status(500).json({ error: "Failed to get after-hours handoff state" });
+  }
+});
+
+// PUT /api/twilio/after-hours-handoff
+router.put("/after-hours-handoff", async (req: Request, res: Response): Promise<any> => {
+  try {
+    const userId = req.user?.id;
+    if (!userId) return res.status(401).json({ message: "Unauthorized" });
+    const { enabled } = req.body;
+    if (typeof enabled !== "boolean") return res.status(400).json({ message: "enabled must be a boolean" });
+    setAfterHoursHandoff(userId, enabled);
+    return res.status(200).json({ enabled });
+  } catch (err) {
+    return res.status(500).json({ error: "Failed to set after-hours handoff state" });
+  }
+});
+
+// GET /api/twilio/ai-handoff/:patientId
+router.get("/ai-handoff/:patientId", async (req: Request, res: Response): Promise<any> => {
+  try {
+    const userId = req.user?.id;
+    if (!userId) return res.status(401).json({ message: "Unauthorized" });
+    const patientId = parseInt(req.params.patientId);
+    if (isNaN(patientId)) return res.status(400).json({ message: "Invalid patientId" });
+    return res.status(200).json({ enabled: getHandoff(userId, patientId) });
+  } catch (err) {
+    return res.status(500).json({ error: "Failed to get AI handoff state" });
+  }
+});
+
+// PUT /api/twilio/ai-handoff/:patientId
+router.put("/ai-handoff/:patientId", async (req: Request, res: Response): Promise<any> => {
+  try {
+    const userId = req.user?.id;
+    if (!userId) return res.status(401).json({ message: "Unauthorized" });
+    const patientId = parseInt(req.params.patientId);
+    if (isNaN(patientId)) return res.status(400).json({ message: "Invalid patientId" });
+    const { enabled } = req.body;
+    if (typeof enabled !== "boolean") return res.status(400).json({ message: "enabled must be a boolean" });
+    setHandoff(userId, patientId, enabled);
+    return res.status(200).json({ enabled });
+  } catch (err) {
+    return res.status(500).json({ error: "Failed to set AI handoff state" });
+  }
+});
+
 // GET /api/twilio/recent-communications
 router.get("/recent-communications", async (req: Request, res: Response): Promise<any> => {
   try {