[CLAUDE] Infra: Mig 43 filter Holiday UNIQUE (Year,Date) by IsDeleted (S45)

Fix drift surfaced by S45 Holiday coverage tests: DB UNIQUE (Year,Date) was unfiltered while handler checks !IsDeleted -> recreating a holiday on a soft-deleted slot threw DbUpdateException(500). Add .HasFilter("[IsDeleted] = 0") matching the 13x project filtered-unique pattern (Catalogs/Contract/PE/Proposal/Budget/WorkflowApps). Soft-deleted slot now reusable per app intent. Flipped Case 7 to assert success-on-reuse. 181 test PASS.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/Backend/SolutionErp.Infrastructure/Persistence/Migrations/20260601064128_FilterHolidayUniqueIndexByIsDeleted.cs

@@ -0,0 +1,39 @@
+using Microsoft.EntityFrameworkCore.Migrations;
+
+#nullable disable
+
+namespace SolutionErp.Infrastructure.Persistence.Migrations
+{
+    /// <inheritdoc />
+    public partial class FilterHolidayUniqueIndexByIsDeleted : Migration
+    {
+        /// <inheritdoc />
+        protected override void Up(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.DropIndex(
+                name: "IX_Holidays_Year_Date",
+                table: "Holidays");
+
+            migrationBuilder.CreateIndex(
+                name: "IX_Holidays_Year_Date",
+                table: "Holidays",
+                columns: new[] { "Year", "Date" },
+                unique: true,
+                filter: "[IsDeleted] = 0");
+        }
+
+        /// <inheritdoc />
+        protected override void Down(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.DropIndex(
+                name: "IX_Holidays_Year_Date",
+                table: "Holidays");
+
+            migrationBuilder.CreateIndex(
+                name: "IX_Holidays_Year_Date",
+                table: "Holidays",
+                columns: new[] { "Year", "Date" },
+                unique: true);
+        }
+    }
+}

src/Backend/SolutionErp.Infrastructure/Persistence/Migrations/ApplicationDbContextModelSnapshot.cs

@@ -2548,7 +2548,8 @@ namespace SolutionErp.Infrastructure.Persistence.Migrations
                     b.HasKey("Id");
 
                     b.HasIndex("Year", "Date")
-                        .IsUnique();
+                        .IsUnique()
+                        .HasFilter("[IsDeleted] = 0");
 
                     b.ToTable("Holidays", (string)null);
                 });