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/Migrations/ApplicationContextModelSnapshot.cs

@@ -696,9 +696,15 @@ namespace GerbilManagerWebAPI.Migrations
                         .IsRequired()
                         .HasColumnType("text");
 
+                    b.Property<string>("NameSearch")
+                        .HasColumnType("text");
+
                     b.Property<string>("Notes")
                         .HasColumnType("text");
 
+                    b.Property<string>("OriginBreeder")
+                        .HasColumnType("text");
+
                     b.Property<Guid?>("OriginContactId")
                         .HasColumnType("uuid");