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/RegionalTaxRule.cs

@@ -0,0 +1,13 @@
+using System.ComponentModel.DataAnnotations.Schema;
+
+namespace Distribution.Models
+{
+    public class RegionalTaxRule : Template
+    {
+        public Guid FK_RegionId { get; set; }
+        [ForeignKey("FK_RegionId")]
+        public Region Region { get; set; }
+
+        public string RuleConfig { get; set; }
+    }
+}