Fix errcheck lint findings in config, docker, and gitea

Handle Fprintf/Close/RemoveAll return values so golangci-lint passes.

Claude-Session: https://claude.ai/code/session_015ur3i7D2azsMAWTSVABApv

internal/docker/docker.go

@@ -69,7 +69,7 @@ func (r *DockerRunner) Run(ctx context.Context, job Job) (Result, error) {
 	if err != nil {
 		return Result{}, err
 	}
-	defer os.RemoveAll(jobDir)
+	defer func() { _ = os.RemoveAll(jobDir) }()
 
 	if job.Timeout > 0 {
 		var cancel context.CancelFunc
@@ -233,7 +233,7 @@ func copyFile(src, dst string) error {
 	if err != nil {
 		return err
 	}
-	defer in.Close()
+	defer func() { _ = in.Close() }()
 	if err := os.MkdirAll(filepath.Dir(dst), 0o700); err != nil {
 		return err
 	}
@@ -241,11 +241,12 @@ func copyFile(src, dst string) error {
 	if err != nil {
 		return err
 	}
-	defer out.Close()
 	if _, err := io.Copy(out, in); err != nil {
+		_ = out.Close()
 		return err
 	}
-	return nil
+	// Close explicitly (not deferred) so a flush error surfaces to the caller.
+	return out.Close()
 }
 
 // copyTree recursively copies a directory tree (regular files, dirs, and