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/Models/Show.cs

@@ -13,7 +13,7 @@ public class Show
     // Link (path/URL/file name) to the pyrotechnic implementation plan
     [Required, MaxLength(500)] public string? PyrotechnicImplementationPlan { get; set; }
     
-    [Required] public int CityId { get; set; }
+    public int? CityId { get; set; }
     public City? City { get; set; }
     
     public List<ShowStaff>? ShowStaffs { get; set; }