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/Endpoints/GerbilEndpoints.cs

@@ -19,6 +19,13 @@ namespace GerbilManagerWebAPI.Endpoints
                 TypedResults.Ok(await db.Gerbils.AsNoTracking()
                     .ToPagedResultAsync(query, ToDto)));
 
+            // GET /gerbils/breeders — distinct non-empty Herkunft values for the Tiere filter dropdown
+            group.MapGet("/breeders", async (ApplicationContext db) =>
+                TypedResults.Ok(await db.Gerbils.AsNoTracking()
+                    .Where(g => g.OriginBreeder != null && g.OriginBreeder != "")
+                    .Select(g => g.OriginBreeder!)
+                    .Distinct().OrderBy(b => b).ToListAsync()));
+
             // GET /gerbils/{id}
             group.MapGet("/{id:guid}", async Task<Results<Ok<GerbilDto>, NotFound>> (Guid id, ApplicationContext db) =>
             {
@@ -87,11 +94,12 @@ namespace GerbilManagerWebAPI.Endpoints
             g.Notes = i.Notes;
             g.ImportSource = i.ImportSource;
             g.ExternalRef = i.ExternalRef;
+            g.OriginBreeder = i.OriginBreeder;
         }
 
         internal static GerbilDto ToDto(Gerbil g) => new(
             g.Id, g.Name, g.Gender, g.Status, g.LitterId, g.OriginContactId, g.ReceiverContactId,
             g.EnclosureId, g.ColorVarietyId, g.DateOfBirth, g.DateOfDeath, g.CauseOfDeath,
-            g.GoHomeDate, g.Genotype, g.Notes, g.ImportSource, g.ExternalRef);
+            g.GoHomeDate, g.Genotype, g.Notes, g.ImportSource, g.ExternalRef, g.OriginBreeder);
     }
 }