fix: add missing migration for anonymousMode column

The column was added to the schema but no migration was created,
causing a PrismaClientKnownRequestError in production.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

prisma/migrations/20260322200000_add_anonymous_mode/migration.sql

@@ -0,0 +1,2 @@
+-- AlterTable
+ALTER TABLE "preferences" ADD COLUMN "anonymousMode" BOOLEAN NOT NULL DEFAULT false;