refactor: modularize email processing logic and improve service structure

- Extract email template handling, rendering, and sending code into `Worker.Services` project.
- Introduce `EmailTemplateService`, `EmailTemplateRenderingService`, and `SendEmailService`.
- Simplify consumer logic by delegating to scoped services.
- Update project dependencies and package references accordingly.

HrynCo.NotificationService.Worker.Services/EmailProcessing/EmailTemplateRenderingService.cs

@@ -0,0 +1,24 @@
+namespace HrynCo.NotificationService.Worker.Services.EmailProcessing;
+
+using System.Text;
+using HrynCo.NotificationService.Contracts.Messages;
+using HrynCo.NotificationService.DAL.Abstract.Templates;
+
+internal sealed class EmailTemplateRenderingService : IEmailTemplateRenderingService
+{
+    public RenderedEmail Render(EmailTemplate template, SendEmailMessageData data)
+    {
+        return new RenderedEmail(
+            Interpolate(template.Subject, data.Variables),
+            Interpolate(template.HtmlBody, data.Variables),
+            Interpolate(template.TextBody, data.Variables));
+    }
+
+    private static string Interpolate(string text, IReadOnlyDictionary<string, string> variables)
+    {
+        var sb = new StringBuilder(text);
+        foreach (var (key, value) in variables)
+            sb.Replace($"{{{{{key}}}}}", value);
+        return sb.ToString();
+    }
+}