feat: Add project-scoped language for unsubscribe footer and contact-facing pages

packages/db/prisma/migrations/20251221114634_add_language_to_project/migration.sql

@@ -0,0 +1,2 @@
+-- AlterTable
+ALTER TABLE "projects" ADD COLUMN     "language" TEXT NOT NULL DEFAULT 'en';

packages/db/prisma/schema.prisma

@@ -57,6 +57,9 @@ model Project {
   // Email Tracking
   tracking TrackingMode @default(ENABLED) // Open and click tracking mode
 
+  // Localization
+  language String @default("en") // Language code for customer-facing content (ISO 639-1)
+
   // Relations
   members     Membership[]
   contacts    Contact[]