feat(expense): add Form990ExpenseLine catalog entity and functional-class constants

API/ROLAC.API/Entities/FunctionalClasses.cs

@@ -0,0 +1,18 @@
+namespace ROLAC.API.Entities;
+
+/// <summary>
+/// The three IRS Form 990 Part IX functional-expense columns. Stored verbatim in
+/// Ministry.DefaultFunctionalClass and Expense.FunctionalClass.
+/// </summary>
+public static class FunctionalClasses
+{
+    public const string Program           = "Program";
+    public const string ManagementGeneral = "ManagementGeneral";
+    public const string Fundraising       = "Fundraising";
+
+    public static readonly IReadOnlyList<string> All = [Program, ManagementGeneral, Fundraising];
+
+    /// <summary>Returns the value if valid, otherwise Program (the safe default).</summary>
+    public static string Normalize(string? value) =>
+        value is not null && All.Contains(value) ? value : Program;
+}