refactor: standardize project names to Musify.* and flatten layout

- Flatten nested per-repo folders into flat Musify.* projects at root
- Rename assemblies/csproj: Domain->Musify.Domain, Application->Musify.Application,
  Infrastructure->Musify.Infrastructure, WebApi->Musify.Api, Worker->Musify.Worker,
  StreamingGateway->Musify.StreamingGateway
- Rename WebApi.* namespaces to Musify.Api.*
- Update all ProjectReference paths and the solution file
- Consolidate stray per-repo dotfiles; add root .gitattributes
- Move flutter client out to _archive

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

Musify.Infrastructure/Persistence/Migrations/20260601195757_RenameTimestampsAndAudit.cs

@@ -0,0 +1,58 @@
+using Microsoft.EntityFrameworkCore.Migrations;
+
+#nullable disable
+
+namespace Musify.Infrastructure.Persistence.Migrations
+{
+    /// <inheritdoc />
+    public partial class RenameTimestampsAndAudit : Migration
+    {
+        /// <inheritdoc />
+        protected override void Up(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.RenameColumn(
+                name: "UpdatedDate",
+                table: "Tracks",
+                newName: "UpdatedAt");
+
+            migrationBuilder.RenameColumn(
+                name: "CreatedDate",
+                table: "Tracks",
+                newName: "CreatedAt");
+
+            migrationBuilder.RenameColumn(
+                name: "UpdatedDate",
+                table: "PlayLists",
+                newName: "UpdatedAt");
+
+            migrationBuilder.RenameColumn(
+                name: "CreatedDate",
+                table: "PlayLists",
+                newName: "CreatedAt");
+        }
+
+        /// <inheritdoc />
+        protected override void Down(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.RenameColumn(
+                name: "UpdatedAt",
+                table: "Tracks",
+                newName: "UpdatedDate");
+
+            migrationBuilder.RenameColumn(
+                name: "CreatedAt",
+                table: "Tracks",
+                newName: "CreatedDate");
+
+            migrationBuilder.RenameColumn(
+                name: "UpdatedAt",
+                table: "PlayLists",
+                newName: "UpdatedDate");
+
+            migrationBuilder.RenameColumn(
+                name: "CreatedAt",
+                table: "PlayLists",
+                newName: "CreatedDate");
+        }
+    }
+}