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>

apps/Backend/src/ai/reschedule-graph.ts

@@ -387,6 +387,7 @@ export async function runRescheduleStep(
     }
 
     // Day not clearly detected — ask again with the specific options
+    const { mon, tue, wed } = getNextWeekDays();
     const fallbacks: Record<string, string> = {
       English:          `Which day works best — ${mon}, ${tue}, or ${wed}?`,
       Spanish:          `¿Qué día le viene mejor — el ${mon}, ${tue} o el ${wed}?`,