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/Endpoints/Show/GetShowEndpoint.cs

@@ -9,7 +9,7 @@ public class GetShowEndpoint(PyroFetesDbContext pyroFetesDbContext) : Endpoint<I
 {
     public override void Configure()
     {
-        Get("/api/shows/{Id}");
+        Get("/shows/{Id}");
         AllowAnonymous();
     }