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/Data/AppDBContext.cs

@@ -56,6 +56,10 @@ public class AppDBContext(DbContextOptions<AppDBContext> options) : DbContext(op
     public DbSet<Claim> Claims { get; set; }
     public DbSet<SupplierClaim> SupplierClaims { get; set; }
     public DbSet<SupplierClaimItem> SupplierClaimItems { get; set; }
+    public DbSet<RegionalTarget> RegionalTargets { get; set; }
+    public DbSet<RegionalBudget> RegionalBudgets { get; set; }
+    public DbSet<RegionAgencyMap> RegionAgencyMaps { get; set; }
+    public DbSet<RegionalTaxRule> RegionalTaxRules { get; set; }
 
     protected override void OnModelCreating(ModelBuilder modelBuilder)
     {
@@ -373,6 +377,28 @@ public class AppDBContext(DbContextOptions<AppDBContext> options) : DbContext(op
             .HasForeignKey(t => t.FK_OrderId)
             .OnDelete(DeleteBehavior.Restrict);
 
+        modelBuilder.Entity<RegionalTarget>()
+            .HasIndex(t => new { t.FK_RegionId, t.Period })
+            .IsUnique();
+
+        modelBuilder.Entity<RegionalBudget>()
+            .HasIndex(b => new { b.FK_RegionId, b.Period })
+            .IsUnique();
+
+        modelBuilder.Entity<RegionAgencyMap>()
+            .HasIndex(m => new { m.FK_RegionId, m.FK_AgencyUserId })
+            .IsUnique();
+
+        modelBuilder.Entity<RegionAgencyMap>()
+            .HasOne(m => m.AgencyUser)
+            .WithMany()
+            .HasForeignKey(m => m.FK_AgencyUserId)
+            .OnDelete(DeleteBehavior.Restrict);
+
+        modelBuilder.Entity<RegionalTaxRule>()
+            .HasIndex(r => r.FK_RegionId)
+            .IsUnique();
+
         Seed.RoleAndPermissionSeed.Apply(modelBuilder);
     }
 }