Add SQL Server distributed cache configuration

Configured the application to use SQL Server as a distributed
cache provider. Added `AddDistributedSqlServerCache` to
`Program.cs` and set up the connection string, schema name,
and table name from the `Cache:SqlServer` configuration
section. This enables persistent and shared caching across
multiple application instances.

EnvelopeGenerator.API/Program.cs

@@ -269,6 +269,15 @@ try
     // Cache options
     builder.Services.Configure<CacheOptions>(config.GetSection(CacheOptions.SectionName));
 
+    // Distributed Cache - SQL Server
+    var sqlCacheConfig = config.GetSection("Cache:SqlServer");
+    builder.Services.AddDistributedSqlServerCache(options =>
+    {
+        options.ConnectionString = sqlCacheConfig["ConnectionString"] ?? connStr;
+        options.SchemaName = sqlCacheConfig["SchemaName"];
+        options.TableName = sqlCacheConfig["TableName"];
+    });
+
     // Envelope generator serives
 #pragma warning disable CS0618 // Type or member is obsolete
     builder.Services