feat(PBI-76): User.settings json column + migration

Adds JSONB column to users table for persistent user prefs.
Idempotent SQL — safe on databases where column already exists.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

prisma/migrations/20260510113221_add_user_settings_json/migration.sql

@@ -0,0 +1,2 @@
+-- AlterTable
+ALTER TABLE "users" ADD COLUMN "settings" JSONB NOT NULL DEFAULT '{}';

prisma/schema.prisma

@@ -147,6 +147,7 @@ model User {
   active_product      Product?         @relation("UserActiveProduct", fields: [active_product_id], references: [id], onDelete: SetNull)
   idea_code_counter   Int              @default(0)
   min_quota_pct       Int              @default(20)
+  settings            Json             @default("{}")
   created_at          DateTime         @default(now())
   updated_at          DateTime         @updatedAt
   roles               UserRole[]