Remove EnvelopeSigningType enum and update envelope logic

Removed EnvelopeSigningType enum and related normalization logic. Added a ReadOnly property to Envelope that uses EnvelopeTypeId to determine read-only status. Envelope type handling now relies on EnvelopeTypeId (int?) instead of the enum.

EnvelopeGenerator.Infrastructure/EGDbContextFactory.cs

@@ -12,7 +12,7 @@ namespace EnvelopeGenerator.Infrastructure
         public EGDbContext CreateDbContext(string[] args)
         {
             var config = new ConfigurationBuilder()
-                .SetBasePath(Directory.GetCurrentDirectory()) // Önemli!
+                .SetBasePath(Directory.GetCurrentDirectory())
                 .AddJsonFile("appsettings.migration.json")
                 .Build();