Expand invoices with full templates, public viewer API, and account holder.

Add invoice template CRUD, key points/accounts, public GET endpoint, and migration 039 for account_holder_name.

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

database/migrations/039_invoice_account_holder.sql

@@ -0,0 +1,7 @@
+-- Add account holder name to invoice / template bank accounts
+
+ALTER TABLE invoice_accounts
+  ADD COLUMN IF NOT EXISTS account_holder_name VARCHAR(255);
+
+ALTER TABLE invoice_template_accounts
+  ADD COLUMN IF NOT EXISTS account_holder_name VARCHAR(255);