feat: persist AI conversation state in DB and fix LangGraph flow bugs

- Replace in-memory Maps in aiHandoffStore with DB-backed async functions
  using new patient_conversation table (stage + aiHandoff per patient)
- Add afterHoursEnabled to ai_settings table (persists across restarts)
- Fix runtime crash in reschedule-graph: mon/tue/wed variables were out
  of scope in the next-week fallback branch (ReferenceError)
- Wire rescheduleGreeting and generalFallback chat templates through to
  LangGraph nodes so user-configured messages take effect
- Add otherNode to reminder-graph to handle unclassified patient replies
  (e.g. "I want another appointment") and route to booking flow
- Fetch chatTemplates once per webhook request instead of per stage

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

packages/db/generated/prisma/schema.prisma

@@ -43,6 +43,7 @@ model User {
   officeContact        OfficeContact?
   procedureTimeslot    ProcedureTimeslot?
   insuranceContacts    InsuranceContact[]
+  patientConversations PatientConversation[]
 }
 
 model Patient {
@@ -75,6 +76,7 @@ model Patient {
   payment           Payment[]
   communications    Communication[]
   documents         PatientDocument[]
+  conversation      PatientConversation?
 
   @@index([insuranceId])
   @@index([createdAt])
@@ -572,9 +574,10 @@ model TwilioSettings {
 }
 
 model AiSettings {
-  id     Int    @id @default(autoincrement())
-  userId Int    @unique
-  apiKey String
+  id                Int     @id @default(autoincrement())
+  userId            Int     @unique
+  apiKey            String
+  afterHoursEnabled Boolean @default(true)
 
   user User @relation(fields: [userId], references: [id], onDelete: Cascade)
 
@@ -627,3 +630,17 @@ model ProcedureTimeslot {
 
   @@map("procedure_timeslot")
 }
+
+model PatientConversation {
+  id        Int      @id @default(autoincrement())
+  patientId Int      @unique
+  userId    Int
+  stage     String   @default("initial")
+  aiHandoff Boolean  @default(true)
+  updatedAt DateTime @updatedAt
+
+  patient Patient @relation(fields: [patientId], references: [id], onDelete: Cascade)
+  user    User    @relation(fields: [userId], references: [id], onDelete: Cascade)
+
+  @@map("patient_conversation")
+}