Add GL account configuration and role mapping entities

- Introduced GlAccountConfig entity for mapping accounting roles to GL account codes.
- Added GlAccountConfigConfiguration for database configuration of the singleton GL account mapping.
- Created GlAccountRole enum to define accounting roles used in GL postings.
- Enhanced error handling with new error codes for unconfigured and invalid GL accounts.
- Updated existing DbContext to include GlAccountConfig DbSet.

Backend/ERPCore/System/Errors/ErrorCodes.cs

@@ -76,6 +76,10 @@ public static class ErrorCodes
     public const string GlServiceUnavailable = "GL_SERVICE_UNAVAILABLE";
     // General Ledger typed internal calls (GRN receipt/payment posting, docs/12 §5's deferred work)
     public const string GlRequestFailed = "GL_REQUEST_FAILED";
+    // A posting flow needs a GL account whose role has no account mapped in gl_account_config.
+    public const string GlAccountNotConfigured = "GL_ACCOUNT_NOT_CONFIGURED";
+    // A submitted GL account mapping names an account GL does not have, or cannot post to.
+    public const string GlAccountInvalid = "GL_ACCOUNT_INVALID";
 
     // Sales invoice payments
     public const string SalesInvoiceNotPayable = "SALES_INVOICE_NOT_PAYABLE";