feat(agent): persist assistant model preference

src/modules/_core/schema.ts

@@ -38,6 +38,7 @@ export const users = pgTable("users", {
   assistantEnabled: boolean("assistant_enabled").notNull().default(false),
   assistantName: text("assistant_name").notNull().default("Assistant"),
   assistantSystemPrompt: text("assistant_system_prompt"),
+  assistantModel: text("assistant_model"),
   defaultEventReminderOffsets: jsonb("default_event_reminder_offsets")
     .notNull()
     .$type<number[]>()