[data] Prevent download deadlock on wide directory trees

[kkozoriz]

Description

Problem

recursiveDownload used a blocking semaphore acquire (sem <- struct{}{}) before
launching each child goroutine. When the directory tree had more direct children
than the semaphore capacity (default: 10), all running goroutines blocked trying
to schedule the next level while holding their semaphore slot - causing a deadlock.

Root Cause

// Before - blocks unconditionally, causing deadlock on wide trees
sem <- struct{}{}
wg.Add(1)
	go func(sp string) {
		defer func() { <-sem; wg.Done() }()
		subErr := recursiveDownload(ctx, sClient, log, sem, url, srcPath+sp, filepath.Join(dstPath, sp))
		if subErr != nil {
			mu.Lock()
			if firstErr == nil {
				firstErr = fmt.Errorf("Download %s: %w", filepath.Join(srcPath, sp), subErr)
			}
			mu.Unlock()		
        }
	}(subPath)

Every active goroutine tried to acquire a new semaphore slot before yielding its
own, which deadlocked as soon as all 10 slots were occupied by tasks that each
needed a free slot to proceed.

Fix

Replace the blocking send with a non-blocking select:

• if a semaphore slot is available → run child in a new goroutine;
• if the semaphore is full → run child synchronously in the current goroutine.

Before fix

After fix