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/schema.prisma

@@ -166,6 +166,7 @@ model invoices {
   tax_date        DateTime?       @db.Date
   paid_date       DateTime?       @db.Date
   issued_by       String?         @db.VarChar(255)
+  billing_text    String?         @db.VarChar(500)
   notes           String?         @db.Text
   internal_notes  String?         @db.Text
   created_at      DateTime?       @default(now()) @db.DateTime(0)