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>

packages/db/generated/prisma/schema.prisma

@@ -42,6 +42,7 @@ model User {
   officeHours          OfficeHours?
   officeContact        OfficeContact?
   procedureTimeslot    ProcedureTimeslot?
+  insuranceContacts    InsuranceContact[]
 }
 
 model Patient {
@@ -61,6 +62,7 @@ model Patient {
   policyHolder      String?
   allergies         String?
   medicalConditions String?
+  preferredLanguage String?                @default("English")
   status            PatientStatus          @default(UNKNOWN)
   userId            Int
   createdAt         DateTime               @default(now())
@@ -592,6 +594,7 @@ model OfficeHours {
 model OfficeContact {
   id               Int     @id @default(autoincrement())
   userId           Int     @unique
+  officeName       String?
   receptionistName String?
   dentistName      String?
   phoneNumber      String?
@@ -603,6 +606,18 @@ model OfficeContact {
   @@map("office_contact")
 }
 
+model InsuranceContact {
+  id          Int      @id @default(autoincrement())
+  userId      Int
+  name        String
+  phoneNumber String?
+  createdAt   DateTime @default(now())
+
+  user User @relation(fields: [userId], references: [id], onDelete: Cascade)
+
+  @@map("insurance_contact")
+}
+
 model ProcedureTimeslot {
   id     Int  @id @default(autoincrement())
   userId Int  @unique