Hotfix v1.3.1: Add missing SystemSettings columns

Fix production database schema mismatch where downloadSource and
primaryFailureFallback columns were missing from SystemSettings table.

Root cause: Migration gap between squashed init migration and production
database setup timing.

Fix: Idempotent migration adds both columns if they don't exist:
- downloadSource TEXT NOT NULL DEFAULT 'soulseek'
- primaryFailureFallback TEXT NOT NULL DEFAULT 'none'

backend/prisma/migrations/20260107000000_add_download_source_columns/migration.sql

@@ -0,0 +1,21 @@
+-- Add downloadSource column if it doesn't exist (idempotent)
+DO $$
+BEGIN
+    IF NOT EXISTS (
+        SELECT 1 FROM information_schema.columns
+        WHERE table_name = 'SystemSettings' AND column_name = 'downloadSource'
+    ) THEN
+        ALTER TABLE "SystemSettings" ADD COLUMN "downloadSource" TEXT NOT NULL DEFAULT 'soulseek';
+    END IF;
+END $$;
+
+-- Add primaryFailureFallback column if it doesn't exist (idempotent)
+DO $$
+BEGIN
+    IF NOT EXISTS (
+        SELECT 1 FROM information_schema.columns
+        WHERE table_name = 'SystemSettings' AND column_name = 'primaryFailureFallback'
+    ) THEN
+        ALTER TABLE "SystemSettings" ADD COLUMN "primaryFailureFallback" TEXT NOT NULL DEFAULT 'none';
+    END IF;
+END $$;