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/DTO/Show/Request/CreateShowDto.cs

@@ -7,5 +7,5 @@ public class CreateShowDto
     public string? Description { get; set; }
     public string? PyrotechnicImplementationPlan { get; set; }
     public DateTime? Date { get; set; }
-    public int CityId { get; set; }
+    public int? CityId { get; set; }
 }