Add data model for region targets, budgets, agency mapping, tax rules

Introduces RegionalTarget, RegionalBudget, RegionAgencyMap, and
RegionalTaxRule entities plus their DbSets and EF constraints, laying
the groundwork for the "beyond CRUD" region management features.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

Distribution/Models/RegionAgencyMap.cs

@@ -0,0 +1,16 @@
+using System.ComponentModel.DataAnnotations.Schema;
+
+namespace Distribution.Models
+{
+    public class RegionAgencyMap : Template
+    {
+        public Guid FK_RegionId { get; set; }
+        [ForeignKey("FK_RegionId")]
+        public Region Region { get; set; }
+
+        /// <summary>User holding the RegionalAgency role that is mapped to this region.</summary>
+        public Guid FK_AgencyUserId { get; set; }
+        [ForeignKey("FK_AgencyUserId")]
+        public User AgencyUser { get; set; }
+    }
+}