Fix slow and inefficient code patterns

[Copilot]

Addresses performance issues including N+1 queries, thread-safety bugs, and unnecessary object allocations.

Changes

• Indexer.saveMusicAsync() - Fixed N+1 query problem by fetching all content IDs in single query, using HashSet for O(1) lookups, and batching saves (500/batch)

• RetryService - Fixed thread-safety (was using shared mutable state), infinite loop when executor returns null, and improper interrupt handling

• Indexer.filterPaths() - Replaced stream().anyMatch() per-path with simple for-loop

• TorrentDownloadManager.startDownload() - Deduplicated torrent client creation logic

• YoutubeCrawler - Made ObjectMapper static (thread-safe, expensive to create)

• Indexer.createMusicEntities() - Parallelized to match createVideoEntities()

Example: N+1 fix

// Before: N queries
songs.stream()
    .filter(song -> !musicRepository.existsByContentId(song.getContentId()))

// After: 1 query + O(1) lookups
Set<String> existingIds = new HashSet<>(musicRepository.findAllContentIds());
songs.stream()
    .filter(song -> !existingIds.contains(song.getContentId()))

Original prompt

Identify and suggest improvements to slow or inefficient code

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