Add EF Core 9 migration to align PostgreSQL test model snapshot

The dependency bump to EF Core / Npgsql 9 changed the computed delete
behavior of the optional Post.Author foreign key. The checked-in model
snapshot (generated under EF Core 8) still recorded OnDelete(Cascade),
so MigrateAsync threw PendingModelChangesWarning and failed PostgreSQL
integration tests. Regenerate the snapshot and add a migration dropping
the cascade to match the current model.

Author: adospace

src/CoreSync.Tests/Migrations/PostgreSQLMigrations/20260619103051_FixAuthorEmailForeignKeyCascade.Designer.cs

@@ -0,0 +1,41 @@
+using Microsoft.EntityFrameworkCore.Migrations;
+
+#nullable disable
+
+namespace CoreSync.Tests.Migrations.PostgreSQLMigrations
+{
+    /// <inheritdoc />
+    public partial class FixAuthorEmailForeignKeyCascade : Migration
+    {
+        /// <inheritdoc />
+        protected override void Up(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.DropForeignKey(
+                name: "FK_Posts_Users_AuthorEmail",
+                table: "Posts");
+
+            migrationBuilder.AddForeignKey(
+                name: "FK_Posts_Users_AuthorEmail",
+                table: "Posts",
+                column: "AuthorEmail",
+                principalTable: "Users",
+                principalColumn: "Email");
+        }
+
+        /// <inheritdoc />
+        protected override void Down(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.DropForeignKey(
+                name: "FK_Posts_Users_AuthorEmail",
+                table: "Posts");
+
+            migrationBuilder.AddForeignKey(
+                name: "FK_Posts_Users_AuthorEmail",
+                table: "Posts",
+                column: "AuthorEmail",
+                principalTable: "Users",
+                principalColumn: "Email",
+                onDelete: ReferentialAction.Cascade);
+        }
+    }
+}

src/CoreSync.Tests/Migrations/PostgreSQLMigrations/20260619103051_FixAuthorEmailForeignKeyCascade.cs

@@ -17,7 +17,7 @@ protected override void BuildModel(ModelBuilder modelBuilder)
         {
 #pragma warning disable 612, 618
             modelBuilder
-                .HasAnnotation("ProductVersion", "8.0.10")
+                .HasAnnotation("ProductVersion", "9.0.17")
                 .HasAnnotation("Relational:MaxIdentifierLength", 63);
 
             NpgsqlModelBuilderExtensions.UseIdentityByDefaultColumns(modelBuilder);
@@ -127,8 +127,7 @@ protected override void BuildModel(ModelBuilder modelBuilder)
                 {
                     b.HasOne("CoreSync.Tests.Data.User", "Author")
                         .WithMany("Posts")
-                        .HasForeignKey("AuthorEmail")
-                        .OnDelete(DeleteBehavior.Cascade);
+                        .HasForeignKey("AuthorEmail");
 
                     b.Navigation("Author");
                 });