feat(tier): Erwerb/Kauf je Tier (Datum, Preis, Notiz)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

GerbilManagerWebAPI/Dtos/AcquisitionDtos.cs

@@ -0,0 +1,20 @@
+namespace GerbilManagerWebAPI.Dtos
+{
+    /// <summary>ERWERB: payload for POST/PUT /acquisitions (acquisition data per animal).</summary>
+    public record AcquisitionInput(
+        Guid? GerbilId,
+        Guid? SourceContactId,
+        DateOnly? Date,
+        decimal? Price,
+        string? Note);
+
+    /// <summary>ERWERB: response DTO for a stored acquisition record.</summary>
+    public record AcquisitionDto(
+        Guid Id,
+        Guid? GerbilId,
+        Guid? SourceContactId,
+        DateOnly? Date,
+        decimal? Price,
+        string? Note,
+        DateTimeOffset CreatedAt);
+}