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

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

GerbilManagerWebAPI/ApplicationContext.cs

@@ -25,6 +25,7 @@ public class ApplicationContext : DbContext
     public DbSet<Request> Requests => Set<Request>();
     public DbSet<MailSettings> MailSettings => Set<MailSettings>();
     public DbSet<Feedback> Feedback => Set<Feedback>();
+    public DbSet<AcquisitionRecord> AcquisitionRecords => Set<AcquisitionRecord>();
 
     // Keep Gerbil.NameSearch in sync on every save (separator-insensitive search key),
     // so it can never drift from Name regardless of which code path mutates the entity.
@@ -212,6 +213,16 @@ public class ApplicationContext : DbContext
             e.HasIndex(f => f.CreatedAt);
         });
 
+        // ERWERB: like Feedback, deliberately relationship-free. GerbilId/SourceContactId
+        // are plain nullable Guid columns (no navigation properties → EF creates NO foreign
+        // key), so the import re-ingest wipe of Gerbils/Contacts never cascades into — or
+        // breaks — acquisition rows. They survive re-ingest, which is the whole point.
+        modelBuilder.Entity<AcquisitionRecord>(e =>
+        {
+            e.Property(a => a.Price).HasPrecision(10, 2);
+            e.HasIndex(a => a.GerbilId);
+        });
+
         // DB-4: German collation on remaining searched/sorted text columns (Npgsql-only).
         if (isNpgsql)
         {