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/Infra/Persistence/Configurations/GlAccountConfigConfiguration.cs

@@ -0,0 +1,51 @@
+using ERPCore.Domain.Entities;
+using Microsoft.EntityFrameworkCore;
+using Microsoft.EntityFrameworkCore.Metadata.Builders;
+
+namespace ERPCore.Infra.Persistence.Configurations;
+
+/// <summary>
+/// Configures the singleton GL account-mapping row. The check constraint is what makes
+/// "singleton" a database guarantee rather than a convention (mirrors <see cref="GrnConfigConfiguration"/>).
+/// Backtick quoting is deliberate - MariaDB rejects the double-quoted form, which crash-looped
+/// startup seeding once already (c97a7e9).
+/// </summary>
+public sealed class GlAccountConfigConfiguration : IEntityTypeConfiguration<GlAccountConfig>
+{
+    public void Configure(EntityTypeBuilder<GlAccountConfig> builder)
+    {
+        builder.ToTable("gl_account_config", t =>
+            t.HasCheckConstraint("ck_gl_account_config_singleton", $"`ConfigId` = {GlAccountConfig.SingletonId}"));
+
+        builder.HasKey(c => c.ConfigId);
+
+        // The id is fixed, never generated — there is exactly one row, seeded by DataSeeder.
+        builder.Property(c => c.ConfigId).ValueGeneratedNever();
+
+        foreach (var code in new[]
+                 {
+                     nameof(GlAccountConfig.InventoryAccountCode),
+                     nameof(GlAccountConfig.VatRecoverableAccountCode),
+                     nameof(GlAccountConfig.AccountsReceivableCode),
+                     nameof(GlAccountConfig.ClearingAccountCode),
+                     nameof(GlAccountConfig.TaxPayableAccountCode),
+                     nameof(GlAccountConfig.AccountsPayableCode),
+                     nameof(GlAccountConfig.SalesRevenueAccountCode),
+                     nameof(GlAccountConfig.SalesDiscountAccountCode),
+                     nameof(GlAccountConfig.InventoryGainAccountCode),
+                     nameof(GlAccountConfig.CogsAccountCode),
+                     nameof(GlAccountConfig.InventoryLossAccountCode),
+                     nameof(GlAccountConfig.CashAccountCode)
+                 })
+        {
+            builder.Property(code).HasMaxLength(GlAccountConfig.CodeMaxLength);
+        }
+
+        // RowVersion is made a concurrency token by the model-wide convention in ErpDbContext.
+
+        builder.HasOne(c => c.UpdatedByUser)
+            .WithMany()
+            .HasForeignKey(c => c.UpdatedBy)
+            .OnDelete(DeleteBehavior.Restrict);
+    }
+}

Backend/ERPCore/Infra/Persistence/ErpDbContext.cs

@@ -48,6 +48,8 @@ public class ErpDbContext : DbContext
     public DbSet<BranchConfig> BranchConfig => Set<BranchConfig>();
     public DbSet<SalesConfig> SalesConfig => Set<SalesConfig>();
     public DbSet<GrnConfig> GrnConfig => Set<GrnConfig>();
+    /// <summary>Singleton row mapping each accounting role to a GL account code.</summary>
+    public DbSet<GlAccountConfig> GlAccountConfig => Set<GlAccountConfig>();
     /// <summary>Singleton row for notification delivery preferences.</summary>
     public DbSet<NotificationConfig> NotificationConfig => Set<NotificationConfig>();
     public DbSet<NotificationConfigModule> NotificationConfigModules => Set<NotificationConfigModule>();