FEAT-8c: import loader — /import/dry-run + /import/execute (gated)

- Additive migration: Gerbil.RawImportData + Litter.PairingCode (Zuchtnummer der Verpaarung).
- ImportService consumes tools/import/output (animals.json + litters.json): litters first
  (authoritative), then conflict-free animals with a DOB; high-confidence (hoch) litter links
  set LitterId, date-only/ambiguous links quarantined; conflicts + stubs never loaded.
  Genotype mapped8locus -> frozen compact string; rawGenotype+unmappedTokens preserved in
  RawImportData; Farbschlag matched to ColorVariety; gender inferred from litter role;
  provenance ImportSource/ExternalRef; idempotent (ExternalRef / Name+Date). Photos copied to store.
- ImportEndpoints: POST /import/dry-run (no writes, Julian-readable report) + /import/execute (gated).

GerbilManagerWebAPI/Models/Litter.cs

@@ -21,5 +21,9 @@ namespace GerbilManagerWebAPI.Models
         /// <summary>Computed ~35 days after Date by default; editable.</summary>
         public DateOnly? ExpectedGoHomeDate { get; set; }
         public string? Notes { get; set; }
+
+        /// <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; }
     }
 }