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.Application/Mixes/GenerateMixesForUser.cs

@@ -304,8 +304,6 @@ namespace Musify.Application.Mixes
                 database.Mixes.RemoveRange(previousMixes);
             }
 
-            var today = DateOnly.FromDateTime(DateTime.UtcNow);
-
             for (var position = 0; position < drafts.Count; position++)
             {
                 var draft = drafts[position];
@@ -314,7 +312,6 @@ namespace Musify.Application.Mixes
                     UserId = userId,
                     Title = draft.Title,
                     Subtitle = draft.Subtitle,
-                    GeneratedOn = today,
                     Position = position
                 };
 

backend/Musify.Application/Mixes/Responses/MixApplicationResponse.cs

@@ -6,7 +6,6 @@ namespace Musify.Application.Mixes.Responses
         Guid Id,
         string Title,
         string? Subtitle,
-        DateOnly GeneratedOn,
         int ItemCount,
         IReadOnlyList<MixItemApplicationResponse> Items)
     {
@@ -20,7 +19,6 @@ namespace Musify.Application.Mixes.Responses
                 mix.Id,
                 mix.Title,
                 mix.Subtitle,
-                mix.GeneratedOn,
                 itemCount,
                 [.. items.Select(MixItemApplicationResponse.FromEntity)]);
     }

backend/Musify.Domain/Entities/Mix.cs

@@ -22,9 +22,6 @@ namespace Musify.Domain.Entities
         [MaxLength(256)]
         public string? Subtitle { get; set; }
 
-        [Required]
-        public required DateOnly GeneratedOn { get; set; }
-
         [Required]
         public int Position { get; set; } = 0;
 

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");