REVIEW-FIXES-2: DB-4 (Deutsche Collation ä/ö/ü) + DB-5 (Litter.ExternalRef)

DB-4 (medium): ICU German collation de-x-icu auf deutschen Textspalten
  Gerbil.Name, Gerbil.NameSearch, Gerbil.OriginBreeder, ColorVariety.Name,
  Contact.Name. Korrekte ä/ö/ü-Sortierung + locale-aware lower() in Postgres.
  Nur bei Npgsql (Database.ProviderName check) — SQLite-Testhost kennt keine
  eigenen Collation-Namen und wuerde EnsureCreated fail lassen.
  Migration AddGermanCollation (ALTER COLUMN ... TYPE text COLLATE de-x-icu).

DB-5 (low): Litter.ExternalRef + filtered unique index + Importer-Anbindung
  Litter.ExternalRef (string?, nullable) = stable import source id (sl.Id).
  Filtered unique index WHERE ExternalRef IS NOT NULL (wie DB-1 fuer Gerbil).
  Migration AddLitterExternalRef.
  Importer (ImportService.cs): setzt ExternalRef = sl.Id beim Litter-Insert;
  prueft existingLitterExtRefSet als PRIMAEREN Idempotenz-Key (ExternalRef),
  Name+Date-Key als Fallback fuer Wuerfe ohne ExternalRef aus frueheren Laeufen.
  +Test DB5_Litter_ExternalRef_set_and_used_for_idempotency (mutiert den Namen
  nach erstem Import, prueft dass Re-Import via ExternalRef erkennt).

GATE: 140/140 C#, has-pending=No.

GerbilManagerWebAPI/Models/Litter.cs

@@ -25,5 +25,10 @@ namespace GerbilManagerWebAPI.Models
         /// <summary>Zuchtnummer der Verpaarung (Wurfchronik col H) — pairing-level code;
         /// litters sharing it are the same Zuchtpaar. Set by the FEAT-8 import.</summary>
         public string? PairingCode { get; set; }
+
+        /// <summary>DB-5: stable import source id (extract.py litter Id). Unique (filtered,
+        /// nulls allowed for manually-entered litters). Primary idempotency key for re-imports;
+        /// Name+Date is the fallback for litters created before this column existed.</summary>
+        public string? ExternalRef { get; set; }
     }
 }