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/shared/schemas/variants/pure/index.ts

@@ -33,3 +33,4 @@ export { OfficeHoursModelSchema } from './OfficeHours.pure';
 export { OfficeContactModelSchema } from './OfficeContact.pure';
 export { InsuranceContactModelSchema } from './InsuranceContact.pure';
 export { ProcedureTimeslotModelSchema } from './ProcedureTimeslot.pure';
+export { PatientConversationModelSchema } from './PatientConversation.pure';