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/EmailTemplateService.cs

@@ -0,0 +1,31 @@
+namespace HrynCo.NotificationService.Worker.Services.EmailProcessing;
+
+using HrynCo.NotificationService.DAL.Abstract.Repositories;
+using HrynCo.NotificationService.DAL.Abstract.Templates;
+
+internal sealed class EmailTemplateService : IEmailTemplateService
+{
+    private readonly IEmailTemplateRepository _templateRepository;
+
+    public EmailTemplateService(IEmailTemplateRepository templateRepository)
+    {
+        _templateRepository = templateRepository;
+    }
+
+    public async Task<EmailTemplate> GetAsync(
+        string serviceName,
+        string templateKey,
+        string? languageCode,
+        CancellationToken cancellationToken)
+    {
+        var lang = string.IsNullOrWhiteSpace(languageCode) ? "en" : languageCode;
+        var template = await _templateRepository.GetAsync(serviceName, templateKey, lang, cancellationToken);
+
+        if (template is null && lang != "en")
+            template = await _templateRepository.GetAsync(serviceName, templateKey, "en", cancellationToken);
+
+        return template
+            ?? throw new InvalidOperationException(
+                $"Template not found: service='{serviceName}' key='{templateKey}' language='{lang}'.");
+    }
+}