Remove /api prefix from all routes and fix CityId FK constraint

- Strip /api prefix from all endpoint routes
- Make Show.CityId nullable (no longer required FK)
- Drop CityId FK constraint and alter column to NULL at startup via raw SQL
- Add migration MakeCityIdNullable for schema consistency
- Update Show DTOs to reflect nullable CityId

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

PyroFetes/Migrations/PyroFetesDbContextModelSnapshot.cs

@@ -813,7 +813,7 @@ namespace PyroFetes.Migrations
 
                     SqlServerPropertyBuilderExtensions.UseIdentityColumn(b.Property<int>("Id"));
 
-                    b.Property<int>("CityId")
+                    b.Property<int?>("CityId")
                         .HasColumnType("int");
 
                     b.Property<DateOnly?>("Date")
@@ -1594,8 +1594,7 @@ namespace PyroFetes.Migrations
                     b.HasOne("PyroFetes.Models.City", "City")
                         .WithMany("Shows")
                         .HasForeignKey("CityId")
-                        .OnDelete(DeleteBehavior.Cascade)
-                        .IsRequired();
+                        .OnDelete(DeleteBehavior.ClientSetNull);
 
                     b.Navigation("City");
                 });