chore: code cleanup and best practices (closes #45)

- Fix gofmt formatting across 4 files
- Add nolint annotations with justifications for all gosec findings
- Resolve all 7 pre-existing linter warnings
- make check now passes cleanly

internal/service/notify/notify.go

@@ -260,7 +260,7 @@ func (svc *Service) sendNtfy(
 	request.Header.Set("Title", title)
 	request.Header.Set("Priority", svc.ntfyPriority(priority))
 
-	resp, err := svc.client.Do(request)
+	resp, err := svc.client.Do(request) //nolint:gosec // URL constructed from trusted config, not user input
 	if err != nil {
 		return fmt.Errorf("failed to send ntfy request: %w", err)
 	}
@@ -352,7 +352,7 @@ func (svc *Service) sendSlack(
 
 	request.Header.Set("Content-Type", "application/json")
 
-	resp, err := svc.client.Do(request)
+	resp, err := svc.client.Do(request) //nolint:gosec // URL from trusted webhook config
 	if err != nil {
 		return fmt.Errorf("failed to send slack request: %w", err)
 	}