feat: add PresignInlineURL method for inline content display (#97)

## Summary

- Add `PresignInlineURL(ctx, key, contentType)` method that generates presigned URLs with `Content-Disposition: inline` and optional `Content-Type` override
- Browsers will render content (e.g. PDFs) directly in iframes instead of triggering download dialogs
- Existing `PresignURL` remains unchanged

## Context

The document-service uses presigned S3 URLs to display PDFs in iframes. Without `Content-Disposition: inline`, the browser triggers a download dialog instead of rendering the PDF.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Reviewed-on: #97

s3.go

@@ -105,6 +105,25 @@ func (s *S3) PresignURL(ctx context.Context, key string) (string, error) {
 	return req.URL, nil
 }
 
+// PresignInlineURL generates a presigned URL that tells the browser to display
+// the content inline rather than triggering a download. The URL is valid for
+// 15 minutes.
+func (s *S3) PresignInlineURL(ctx context.Context, key string, contentType string) (string, error) {
+	input := &s3.GetObjectInput{
+		Bucket:                     aws.String(s.bucket),
+		Key:                        aws.String(key),
+		ResponseContentDisposition: aws.String("inline"),
+	}
+	if contentType != "" {
+		input.ResponseContentType = aws.String(contentType)
+	}
+	req, err := s.presigner.PresignGetObject(ctx, input, s3.WithPresignExpires(15*time.Minute))
+	if err != nil {
+		return "", err
+	}
+	return req.URL, nil
+}
+
 // New creates a new S3 storage instance using the upload manager
 // This loads AWS config from the default locations and is suitable for most use cases
 func New(bucket string) (*S3, error) {