feat: add audiobook settings columns to schema + migration

backend/prisma/migrations/20260514000000_add_audiobook_support/migration.sql

@@ -0,0 +1,4 @@
+-- AlterTable
+ALTER TABLE "system_settings" ADD COLUMN "audiobook_request_cost" INTEGER NOT NULL DEFAULT 300;
+ALTER TABLE "system_settings" ADD COLUMN "audiobook_credits_per_minute" INTEGER NOT NULL DEFAULT 1;
+ALTER TABLE "system_settings" ADD COLUMN "audiobook_min_listen_minutes" INTEGER NOT NULL DEFAULT 5;

backend/prisma/schema.prisma

@@ -117,6 +117,9 @@ model SystemSettings {
   bonusMultiplierActive Boolean @default(false) @map("bonus_multiplier_active")
   bonusMultiplier      Decimal @default(2.0) @map("bonus_multiplier") @db.Decimal(3, 2)
   mintingPaused Boolean @default(false) @map("minting_paused")
+  audiobookRequestCost Int @default(300) @map("audiobook_request_cost")
+  audiobookCreditsPerMinute Int @default(1) @map("audiobook_credits_per_minute")
+  audiobookMinListenMinutes Int @default(5) @map("audiobook_min_listen_minutes")
   updatedAt DateTime @updatedAt @map("updated_at")
   updatedBy String? @map("updated_by")
   @@map("system_settings")