SEARCH-1: separator-insensitive NameSearch + OriginBreeder (Herkunft) filter

- Gerbil.NameSearch (lowercase, strip whitespace/.-_) auto-synced in ApplicationContext
  .SaveChanges; Gridify-filterable so 'clan kleine chaoten' matches 'Clan-Kleine-Chaoten'
  (client strips separators the same way). GerbilSearch.Normalize shared helper.
- Gerbil.OriginBreeder (free-text Herkunft) on DTO+Input, set by the FEAT-8 import from the
  source Zucht (imported animals have no OriginContact). Gridify-filterable.
- GET /gerbils/breeders: distinct non-empty OriginBreeder values for the Herkunft dropdown.
- Migration AddSearchFields (+ NameSearch backfill SQL for existing rows).

GerbilManagerWebAPI/Dtos/ApiDtos.cs

@@ -24,7 +24,8 @@ namespace GerbilManagerWebAPI.Dtos
         string? Genotype,
         string? Notes,
         string? ImportSource,
-        string? ExternalRef);
+        string? ExternalRef,
+        string? OriginBreeder);
 
     public record LitterDto(
         Guid Id,
@@ -69,7 +70,8 @@ namespace GerbilManagerWebAPI.Dtos
         string? Genotype,
         string? Notes,
         string? ImportSource,
-        string? ExternalRef);
+        string? ExternalRef,
+        string? OriginBreeder);
 
     public record LitterInput(
         string Name,