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>

src/schemas/invoices.schema.ts

@@ -27,6 +27,7 @@ export const CreateInvoiceSchema = z.object({
   due_date: z.string().nullish(),
   tax_date: z.string().nullish(),
   issued_by: z.string().nullish(),
+  billing_text: z.string().nullish(),
   notes: z.string().nullish(),
   internal_notes: z.string().nullish(),
   items: z.array(InvoiceItemSchema).optional(),