Add track coauthors with invitations, acceptance and coauthor-aware track responses

Owners invite existing users to coauthor a track; invitees accept or reject,
and either side can remove a coauthor. Only accepted coauthors appear in track
responses and in the coauthor's public track list. The owner also receives the
full invitation list with statuses on the user tracks response.

backend/Core/Musify.Infrastructure/Persistence/Database.cs

@@ -59,6 +59,8 @@ namespace Musify.Infrastructure.Persistence
 
         public DbSet<UserFollow> UserFollows => Set<UserFollow>();
 
+        public DbSet<TrackCoauthor> TrackCoauthors => Set<TrackCoauthor>();
+
         public async Task<IDatabaseTransaction> BeginTransactionAsync(System.Data.IsolationLevel isolationLevel, CancellationToken cancellationToken)
         {
             var transaction = await Database.BeginTransactionAsync(isolationLevel, cancellationToken);

backend/Core/Musify.Infrastructure/Persistence/Migrations/20260925001154_AddTrackCoauthors.cs

@@ -0,0 +1,61 @@
+using System;
+using Microsoft.EntityFrameworkCore.Migrations;
+
+#nullable disable
+
+namespace Musify.Infrastructure.Persistence.Migrations
+{
+    /// <inheritdoc />
+    public partial class AddTrackCoauthors : Migration
+    {
+        /// <inheritdoc />
+        protected override void Up(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.CreateTable(
+                name: "TrackCoauthors",
+                columns: table => new
+                {
+                    Id = table.Column<Guid>(type: "uuid", nullable: false),
+                    TrackId = table.Column<Guid>(type: "uuid", nullable: false),
+                    UserId = table.Column<long>(type: "bigint", nullable: false),
+                    Status = table.Column<int>(type: "integer", nullable: false),
+                    InvitedAt = table.Column<DateTime>(type: "timestamp with time zone", nullable: false),
+                    RespondedAt = table.Column<DateTime>(type: "timestamp with time zone", nullable: true)
+                },
+                constraints: table =>
+                {
+                    table.PrimaryKey("PK_TrackCoauthors", x => x.Id);
+                    table.ForeignKey(
+                        name: "FK_TrackCoauthors_Tracks_TrackId",
+                        column: x => x.TrackId,
+                        principalTable: "Tracks",
+                        principalColumn: "Id",
+                        onDelete: ReferentialAction.Cascade);
+                    table.ForeignKey(
+                        name: "FK_TrackCoauthors_User_UserId",
+                        column: x => x.UserId,
+                        principalTable: "User",
+                        principalColumn: "Id",
+                        onDelete: ReferentialAction.Cascade);
+                });
+
+            migrationBuilder.CreateIndex(
+                name: "IX_TrackCoauthors_TrackId_UserId",
+                table: "TrackCoauthors",
+                columns: new[] { "TrackId", "UserId" },
+                unique: true);
+
+            migrationBuilder.CreateIndex(
+                name: "IX_TrackCoauthors_UserId_Status",
+                table: "TrackCoauthors",
+                columns: new[] { "UserId", "Status" });
+        }
+
+        /// <inheritdoc />
+        protected override void Down(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.DropTable(
+                name: "TrackCoauthors");
+        }
+    }
+}

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

@@ -467,6 +467,37 @@ namespace Musify.Infrastructure.Persistence.Migrations
                     b.ToTable("Tracks");
                 });
 
+            modelBuilder.Entity("Musify.Domain.Entities.TrackCoauthor", b =>
+                {
+                    b.Property<Guid>("Id")
+                        .ValueGeneratedOnAdd()
+                        .HasColumnType("uuid");
+
+                    b.Property<DateTime>("InvitedAt")
+                        .HasColumnType("timestamp with time zone");
+
+                    b.Property<DateTime?>("RespondedAt")
+                        .HasColumnType("timestamp with time zone");
+
+                    b.Property<int>("Status")
+                        .HasColumnType("integer");
+
+                    b.Property<Guid>("TrackId")
+                        .HasColumnType("uuid");
+
+                    b.Property<long>("UserId")
+                        .HasColumnType("bigint");
+
+                    b.HasKey("Id");
+
+                    b.HasIndex("TrackId", "UserId")
+                        .IsUnique();
+
+                    b.HasIndex("UserId", "Status");
+
+                    b.ToTable("TrackCoauthors");
+                });
+
             modelBuilder.Entity("Musify.Domain.Entities.TrackLike", b =>
                 {
                     b.Property<Guid>("Id")
@@ -989,6 +1020,25 @@ namespace Musify.Infrastructure.Persistence.Migrations
                         .IsRequired();
                 });
 
+            modelBuilder.Entity("Musify.Domain.Entities.TrackCoauthor", b =>
+                {
+                    b.HasOne("Musify.Domain.Entities.Track", "Track")
+                        .WithMany("Coauthors")
+                        .HasForeignKey("TrackId")
+                        .OnDelete(DeleteBehavior.Cascade)
+                        .IsRequired();
+
+                    b.HasOne("Musify.Domain.Entities.User", "User")
+                        .WithMany()
+                        .HasForeignKey("UserId")
+                        .OnDelete(DeleteBehavior.Cascade)
+                        .IsRequired();
+
+                    b.Navigation("Track");
+
+                    b.Navigation("User");
+                });
+
             modelBuilder.Entity("Musify.Domain.Entities.TrackLike", b =>
                 {
                     b.HasOne("Musify.Domain.Entities.Track", "Track")
@@ -1068,6 +1118,8 @@ namespace Musify.Infrastructure.Persistence.Migrations
                 {
                     b.Navigation("AlbumTracks");
 
+                    b.Navigation("Coauthors");
+
                     b.Navigation("ListeningHistories");
 
                     b.Navigation("PlayListTracks");

backend/Core/Musify.Infrastructure/Persistence/ModelConfiguration.cs

@@ -122,6 +122,25 @@ namespace Musify.Infrastructure.Persistence
                 .HasIndex(trackTag => new { trackTag.TrackId, trackTag.Tag })
                 .IsUnique();
 
+            modelBuilder.Entity<TrackCoauthor>()
+                .HasOne(coauthor => coauthor.Track)
+                .WithMany(track => track.Coauthors)
+                .HasForeignKey(coauthor => coauthor.TrackId)
+                .OnDelete(DeleteBehavior.Cascade);
+
+            modelBuilder.Entity<TrackCoauthor>()
+                .HasOne(coauthor => coauthor.User)
+                .WithMany()
+                .HasForeignKey(coauthor => coauthor.UserId)
+                .OnDelete(DeleteBehavior.Cascade);
+
+            modelBuilder.Entity<TrackCoauthor>()
+                .HasIndex(coauthor => new { coauthor.TrackId, coauthor.UserId })
+                .IsUnique();
+
+            modelBuilder.Entity<TrackCoauthor>()
+                .HasIndex(coauthor => new { coauthor.UserId, coauthor.Status });
+
             modelBuilder.Entity<UserFollow>()
                 .HasOne(follow => follow.Follower)
                 .WithMany()