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/package.json

@@ -1,5 +1,5 @@
 {
-  "name": "prisma-client-15a1f883083e7bed207e0ace797742e2241c3e7ca8304117cb9604fe5f882a3f",
+  "name": "prisma-client-841b8b7214f98d624249fcce8221ef527e6df11b7ccf3685b686d6606451965c",
   "main": "index.js",
   "types": "index.d.ts",
   "browser": "default.js",