feat: editable billing text on invoices

- Added billing_text column to invoices table (VARCHAR 500)
- Prisma migration: 20260323_add_billing_text
- Form field on invoice create page with placeholder
- PDF uses billing_text, falls back to default translation
- Stored on create and editable on draft invoices

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

prisma/migrations/20260323_add_billing_text/migration.sql

@@ -0,0 +1,2 @@
+-- Add billing_text column to invoices table
+ALTER TABLE `invoices` ADD COLUMN `billing_text` VARCHAR(500) NULL AFTER `issued_by`;