feat: prefer base64 inline image for summary, fall back to S3

Send the screenshot to the multimodal model inline as a base64 data URL
so summarisation no longer requires S3 to be configured or publicly
reachable. Only when the PNG exceeds the provider MaxInlineBytes limit
do we upload to S3, verify the public URL is reachable, and delete the
temporary object once done. Oversized images without S3 now surface a
clear hint instead of a hard S3 requirement.

internal/infrastructure/oss/s3.go

@@ -101,6 +101,20 @@ func (p *S3Provider) BuildPublicURL(key string) string {
 	return endpoint + "/" + p.cfg.Bucket + "/" + key
 }
 
+// Delete removes a previously uploaded object by key.
+//
+// The summary pipeline uses this to clean up the temporary screenshot it had
+// to upload only so a remote multimodal model could fetch it via public URL.
+func (p *S3Provider) Delete(ctx context.Context, key string) error {
+	if _, err := p.client.DeleteObject(ctx, &s3.DeleteObjectInput{
+		Bucket: awsv2.String(p.cfg.Bucket),
+		Key:    awsv2.String(key),
+	}); err != nil {
+		return fmt.Errorf("s3 delete object: %w", err)
+	}
+	return nil
+}
+
 // TestConnection performs a small put + delete to verify credentials and
 // bucket reachability. Used by the "Test connection" button in settings.
 func (p *S3Provider) TestConnection(ctx context.Context) error {