Refactor email template commands and controller logic

- Updated namespace for `ResetEmailTemplateCommand` and added a constructor for flexible initialization.
- Introduced `ChangedWhen` property in `UpdateEmailTemplateCommand` to track modification time.
- Refactored `UpdateEmailTemplateCommandHandler` for improved email template retrieval logic.
- Modified `EmailTemplateController` method signatures and logic for clarity and consistency.
- Added `EmailTemplate` entry in `appsettings.json` for database trigger configuration.

EnvelopeGenerator.Application/EmailTemplates/Commands/Update/UpdateEmailTemplateCommand.cs

@@ -20,4 +20,10 @@ public record UpdateEmailTemplateCommand(string? Body = null, string? Subject =
     /// </param>
     [JsonIgnore]
     public EmailTemplateQuery? EmailTemplateQuery { get; set; }
+
+    /// <summary>
+    /// 
+    /// </summary>
+    [JsonIgnore]
+    public DateTime ChangedWhen { get; init; } = DateTime.Now;
 }