update on production db

Backend/ERPCore/Program.cs

@@ -36,8 +36,14 @@ builder.Services.AddControllers()
 // commands (`dotnet ef migrations add`) and startup depend on the database being reachable.
 // MariaDbServerVersion — not MySqlServerVersion — because the target is MariaDB 10.11, and
 // Pomelo changes its SQL translation based on which it is given.
-var connectionString = builder.Configuration.GetConnectionString("DefaultConnection")
-    ?? throw new InvalidOperationException("ConnectionStrings:DefaultConnection is not configured.");
+// IsNullOrWhiteSpace, not just null: appsettings.json ships the key with an empty value so the
+// shape is self-documenting, and each environment supplies the real one. A blank must fail here
+// with this message rather than reaching UseMySql, which would report an opaque parse error.
+var connectionString = builder.Configuration.GetConnectionString("DefaultConnection");
+if (string.IsNullOrWhiteSpace(connectionString))
+    throw new InvalidOperationException(
+        "ConnectionStrings:DefaultConnection is not configured. Set it in appsettings.<Environment>.json "
+        + "or as the ConnectionStrings__DefaultConnection environment variable.");
 
 builder.Services.AddDbContext<ErpDbContext>(o =>
 {