Feature request: Preserve Symlink completed-file action for virtual filesystem download clients

[sawdoctor]

## Summary

I'd like to propose a new completed-file action called **Preserve Symlink**.

The use case is download clients / virtual filesystems such as nzbDAV / InfiniDysk, where a completed audiobook may already be exposed as a filesystem symlink to virtual storage rather than as a normal local file.

Listenarr's existing Move / Copy / Hardlink-Copy actions are not safe for this workflow because copying or falling back to a copy can materialize the entire virtual file onto local storage.

## Proposed behaviour

When **Preserve Symlink** is selected:

- The completed source must be a symbolic link.
- The symlink target must be absolute.
- Listenarr recreates the same symlink at the organised library destination.
- A regular source file is rejected.
- A broken source symlink is rejected.
- An existing different destination is not overwritten.
- There is **no File.Copy, stream-copy, hardlink or move fallback**.
- Destination planning should avoid content-equivalence checks that would dereference/read the virtual audiobook payload.
- On success the destination is returned/registered normally by Listenarr.

The important safety property is that selecting Preserve Symlink can never silently turn into a full file copy.

## Why I think this should be generic

Although I developed and tested it with nzbDAV / InfiniDysk, the implementation does not contain any InfiniDysk-specific paths or APIs.

It should therefore also be useful for other download clients or storage systems that expose completed files as symlinks.

Example:

```text
Download client
    ↓
/mnt/nzbdav/completed-symlinks/audio/book.m4b
    ↓ symlink to
/mnt/nzbdav/.ids/<id>
    ↓
Listenarr Preserve Symlink
    ↓
/audiobooks/Author/Book/book.m4b
    ↓ same symlink target
/mnt/nzbdav/.ids/<id>

Current implementation / testing

I have a working implementation in my fork:

https://github.com/sawdoctor/Listenarr/tree/infinidysk-compat

The feature has been tested with:

• real nzbDAV / InfiniDysk audiobook imports
• multi-file audiobook imports
• Audiobookshelf reading the resulting library symlinks
• seeking/playback through the virtual files
• source-symlink → destination-symlink unit testing
• regular-file rejection testing
• frontend support for selecting preserve-symlink

Targeted tests currently pass:

• frontend: 10 passed
• Preserve Symlink backend tests: 2 passed, 0 failed

The implementation deliberately fails closed if the completed source is not a symlink.

Upstream PR

If this is something you would accept upstream, I'm happy to prepare a clean PR rebased onto the latest canary.

My current fork branch also contains two unrelated fixes found during testing, so I would split those out and submit Preserve Symlink only in its own PR in accordance with the contribution guidelines.