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/Import/ImportService.cs

@@ -176,6 +176,7 @@ namespace GerbilManagerWebAPI.Import
                         Genotype = ComposeGenotype(a.Genotype),
                         ImportSource = ImportSourceTag,
                         ExternalRef = a.Id,
+                        OriginBreeder = string.IsNullOrWhiteSpace(a.Zucht) ? null : a.Zucht.Trim(),
                         RawImportData = JsonSerializer.Serialize(new
                         {
                             a.Genotype.RawGenotype,