fix: honor qBittorrent multi-file save paths

[thedancingdeveloper]

Summary

• interpret qBittorrent savepath as a payload root and retain the torrent-name directory for multi-file torrents
• preserve that root through JSON session persistence/restart
• return qBittorrent-compatible save paths, content paths, and multi-file names
• add read-only end-to-end coverage for existing multi-file payloads and persistence reload

Live canary finding

The Phase 1 migration canary verified a completed single-file item, while a completed multi-file item stayed at zero because its files were sought directly below the save path. Docker and application read-only guards held; full payload manifests and all 32 sampled hashes remained unchanged. The three destination metadata records were then forgotten with deleteFiles=false.

Validation

• cargo fmt --all -- --check
• cargo check --workspace --exclude rtbit-desktop --no-default-features --features default-tls
• cargo test --workspace --exclude rtbit-desktop --no-default-features --features default-tls
• cargo clippy --workspace --exclude rtbit-desktop --no-default-features --features default-tls -- -D warnings
• cargo build --release -p rtbit --no-default-features --features default-tls