refactor: drop dead code left after the worktree move

ChipGroup is orphaned since the explore rewrite dropped the filter chips.
Mix.GeneratedOn was written on every generation and never read, and it
duplicated CreatedAt, which IAuditable already sets at the same moment.

backend/Musify.Infrastructure/Persistence/Migrations/20260721162102_DropMixGeneratedOn.cs

@@ -0,0 +1,30 @@
+using System;
+using Microsoft.EntityFrameworkCore.Migrations;
+
+#nullable disable
+
+namespace Musify.Infrastructure.Persistence.Migrations
+{
+    /// <inheritdoc />
+    public partial class DropMixGeneratedOn : Migration
+    {
+        /// <inheritdoc />
+        protected override void Up(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.DropColumn(
+                name: "GeneratedOn",
+                table: "Mixes");
+        }
+
+        /// <inheritdoc />
+        protected override void Down(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.AddColumn<DateOnly>(
+                name: "GeneratedOn",
+                table: "Mixes",
+                type: "date",
+                nullable: false,
+                defaultValue: new DateOnly(1, 1, 1));
+        }
+    }
+}

backend/Musify.Infrastructure/Persistence/Migrations/DatabaseModelSnapshot.cs

@@ -325,9 +325,6 @@ namespace Musify.Infrastructure.Persistence.Migrations
                     b.Property<DateTime>("CreatedAt")
                         .HasColumnType("timestamp with time zone");
 
-                    b.Property<DateOnly>("GeneratedOn")
-                        .HasColumnType("date");
-
                     b.Property<int>("Position")
                         .HasColumnType("integer");