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/routes/admin/invoices-pdf.ts

@@ -880,7 +880,7 @@ ${indentCSS}
   </div>
 
   <!-- Polozky -->
-  <div class="billing-label">${escapeHtml(t.billing)}</div>
+  <div class="billing-label">${escapeHtml(invoice.billing_text || t.billing)}</div>
   <table class="items">
     <thead>
       <tr>