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/ai-settings.ts

@@ -36,4 +36,30 @@ router.put("/settings", async (req: Request, res: Response): Promise<any> => {
   }
 });
 
+// GET /api/ai/chat-templates
+router.get("/chat-templates", async (req: Request, res: Response): Promise<any> => {
+  try {
+    const userId = req.user?.id;
+    if (!userId) return res.status(401).json({ message: "Unauthorized" });
+    const templates = await storage.getAiChatTemplates(userId);
+    return res.status(200).json(templates);
+  } catch (err) {
+    return res.status(500).json({ error: "Failed to fetch AI chat templates", details: String(err) });
+  }
+});
+
+// PUT /api/ai/chat-templates
+router.put("/chat-templates", async (req: Request, res: Response): Promise<any> => {
+  try {
+    const userId = req.user?.id;
+    if (!userId) return res.status(401).json({ message: "Unauthorized" });
+    const { reminderGreeting, newPatientGreeting, generalFallback } = req.body;
+    await storage.saveAiChatTemplates(userId, { reminderGreeting, newPatientGreeting, generalFallback });
+    const updated = await storage.getAiChatTemplates(userId);
+    return res.status(200).json(updated);
+  } catch (err) {
+    return res.status(500).json({ error: "Failed to save AI chat templates", details: String(err) });
+  }
+});
+
 export default router;