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

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

GerbilManagerWebAPI/Models/AcquisitionRecord.cs

@@ -0,0 +1,38 @@
+using System.ComponentModel.DataAnnotations;
+
+namespace GerbilManagerWebAPI.Models
+{
+    /// <summary>
+    /// ERWERB: how/when an animal was acquired (purchase date, price, note) — sourced from
+    /// RennmausPro's <c>herktier_tb</c> (_TID/_DATE/_PRICE/_BEM). The seller is already linked
+    /// via Gerbil.OriginContactId; this record adds the date/price/note around that.
+    ///
+    /// Decoupled from the rest of the model ON PURPOSE — GerbilId/SourceContactId are plain
+    /// nullable Guid columns (NOT enforced foreign keys), so the import re-ingest wipe
+    /// (IngestResolvedService, which deletes Gerbils/Contacts) never cascades into — or
+    /// breaks — acquisition rows. They survive re-ingest, mirroring the Feedback pattern.
+    /// </summary>
+    public class AcquisitionRecord
+    {
+        [Key]
+        public Guid Id { get; set; }
+
+        /// <summary>Loose reference (no FK) to the acquired gerbil.</summary>
+        public Guid? GerbilId { get; set; }
+
+        /// <summary>Loose reference (no FK) to the source/seller contact, if known.</summary>
+        public Guid? SourceContactId { get; set; }
+
+        /// <summary>When the animal was acquired (purchase date).</summary>
+        public DateOnly? Date { get; set; }
+
+        /// <summary>Acquisition price (purchase cost), if recorded.</summary>
+        public decimal? Price { get; set; }
+
+        /// <summary>Free-text note about the acquisition.</summary>
+        public string? Note { get; set; }
+
+        /// <summary>Server-side creation time.</summary>
+        public DateTimeOffset CreatedAt { get; set; }
+    }
+}