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/Migrations/20260606052952_AddImportProvenanceFields.cs

@@ -0,0 +1,38 @@
+using Microsoft.EntityFrameworkCore.Migrations;
+
+#nullable disable
+
+namespace GerbilManagerWebAPI.Migrations
+{
+    /// <inheritdoc />
+    public partial class AddImportProvenanceFields : Migration
+    {
+        /// <inheritdoc />
+        protected override void Up(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.AddColumn<string>(
+                name: "PairingCode",
+                table: "Litters",
+                type: "text",
+                nullable: true);
+
+            migrationBuilder.AddColumn<string>(
+                name: "RawImportData",
+                table: "Gerbils",
+                type: "text",
+                nullable: true);
+        }
+
+        /// <inheritdoc />
+        protected override void Down(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.DropColumn(
+                name: "PairingCode",
+                table: "Litters");
+
+            migrationBuilder.DropColumn(
+                name: "RawImportData",
+                table: "Gerbils");
+        }
+    }
+}