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

Musify.Infrastructure/Persistence/DatabaseDesignTimeFactory.cs

@@ -0,0 +1,29 @@
+using Microsoft.EntityFrameworkCore;
+using Microsoft.EntityFrameworkCore.Design;
+using Microsoft.Extensions.Configuration;
+using Musify.Infrastructure.Configuration;
+
+namespace Musify.Infrastructure.Persistence
+{
+    public class DatabaseDesignTimeFactory : IDesignTimeDbContextFactory<Database>
+    {
+        public Database CreateDbContext(string[] args)
+        {
+            var configuration = new ConfigurationBuilder()
+                .SetBasePath(Directory.GetCurrentDirectory())
+                .AddJsonFile("DesignSettings.json")
+                .AddUserSecrets<DatabaseDesignTimeFactory>()
+                .Build();
+
+            var databaseConfiguration = configuration
+                .GetRequiredSection(DatabaseConfiguration.SectionName)
+                .Get<DatabaseConfiguration>()
+                ?? throw new InvalidOperationException($"{DatabaseConfiguration.SectionName} configuration section not found.");
+
+            var optionsBuilder = new DbContextOptionsBuilder<Database>();
+            optionsBuilder.UseNpgsql(databaseConfiguration.ConnectionString);
+
+            return new Database(databaseConfiguration);
+        }
+    }
+}