feat(envelopes): add support for SQLExecutor or repository when creating envelopes

- Refactored `CreateEnvelopeCommandHandler` to resolve dependencies via `IServiceProvider`
- Added `IRepository<Envelope>` to allow repository-based envelope creation
- Updated handler logic to choose between `IEnvelopeExecutor` and repository based on `UseSQLExecutor` flag

EnvelopeGenerator.Application/Envelopes/Commands/CreateEnvelopeCommand.cs

@@ -36,7 +36,10 @@ public record CreateEnvelopeCommand : IRequest<EnvelopeDto?>
     public int UserId { get; set; }
 
     /// <summary>
-    /// if true, use <seealso cref="IEnvelopeExecutor"/>; otherwise, use <seealso cref="IRepository{Envelope}"/>
+    /// Determines which component is used for envelope processing.
+    /// When <c>true</c>, processing is delegated to <see cref="IEnvelopeExecutor"/>; 
+    /// when <c>false</c>, <see cref="IRepository{Envelope}"/> is used instead. 
+    /// Note: <see cref="IRepository{Envelope}"/> should only be used in testing scenarios.
     /// </summary>
     [JsonIgnore]
     [NotMapped]