make audiobook organization recursive

[sgerner]

This change makes audiobook organization preserve nested directory trees instead of flattening only the top level.

What changed:

• Added a recursive directory mover for audiobook imports.
• Kept the existing single-file audiobook path unchanged.
• Added regression coverage for a nested CD1/ and CD2/Extras/ layout.
• Preserved the guard that avoids creating an empty destination directory when the source path is missing.

Why:

• Some audiobook torrents contain nested disc or extras folders.
• The previous implementation only moved direct children and did not recurse, which broke those imports.

Validation:

• go test ./internal/organize
• Rebuilt the local librarr:prowlarr-audiobooks image.
• Recreated the running librarr container.

[JeremiahM37]

Recursive mover looks good. Two things:

1. One symlink concern inline.
2. This branch is stacked on #52 — the audiobookbay.go and audiobookbay_test.go diffs in this PR are the same as #52's. Once #52 lands please rebase onto main so that delta falls out.

[JeremiahM37]

WalkDir doesn't follow directory symlinks (good), but a symlinked file inside a torrent will get dereferenced by moveFile's copy+delete fallback — that path uses ReadFile/WriteFile, so a symlink pointing at /etc/passwd would get copied into the audiobook library. Worth a d.Type()&os.ModeSymlink != 0 skip in the walk before moveFile is called.