feat: per-user assistant name and system prompt customization

Each user can rename the AI assistant and edit their own system prompt in Settings.

src/modules/_core/schema.ts

@@ -36,6 +36,8 @@ export const users = pgTable("users", {
   notifInApp: boolean("notif_inapp").notNull().default(true),
   notifNtfy: boolean("notif_ntfy").notNull().default(false),
   assistantEnabled: boolean("assistant_enabled").notNull().default(false),
+  assistantName: text("assistant_name").notNull().default("Assistant"),
+  assistantSystemPrompt: text("assistant_system_prompt"),
   defaultEventReminderOffsets: jsonb("default_event_reminder_offsets")
     .notNull()
     .$type<number[]>()