Add platform invoices API with item templates and optional name.

Super admins can manage predefined invoice lines and issue invoices to businesses; schema is ready for future business-scoped use.

Co-authored-by: Cursor <cursoragent@cursor.com>

database/migrations/037_invoice_name.sql

@@ -0,0 +1,4 @@
+-- Meshkee CMS — optional invoice name
+
+ALTER TABLE invoices
+  ADD COLUMN IF NOT EXISTS name VARCHAR(255);