fix: support single-entry multifile save paths

[thedancingdeveloper]

Summary

• classify multi-file torrents from metainfo instead of file count
• preserve qBittorrent save-path root semantics for single-entry multi-file torrents
• persist the output-folder root in the PostgreSQL backend
• cover the discovered layout with a read-only verification test

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 --profile release-github
• targeted read-only single-entry multi-file test

The live migration inventory contains three torrents with this metainfo shape.