PROFILEPHOTO: profilePhotoUrl computed field on GerbilDto

GET /gerbils/{id}: loads first photo by sortOrder (2 queries, no N+1).
GET /gerbils list: batch-loads first photos for the page in one IN query.
profilePhotoUrl = /photos/files/{fileName} or null. No migration.
3 new tests: no-photo=null, sortOrder priority, list batch.

GerbilManagerWebAPI/Dtos/ApiDtos.cs

@@ -29,7 +29,8 @@ namespace GerbilManagerWebAPI.Dtos
         List<string> CharacterTraits,
         string? CharacterNote,
         bool? IsDeaf,
-        bool IsResident);
+        bool IsResident,
+        string? ProfilePhotoUrl);
 
     public record LitterDto(
         Guid Id,