fix(importer): import only torrent file list, not whole download dir (closes #903) - #933

Closes #903.

Reporter's repro (Bclark117)

• Transmission with a shared download directory (e.g. /data/downloads); PathRemap configured per client (/data:/downloads).
• The user seeds several books in that one directory; each is a single loose .m4b file with no containing subfolder.
• A new completed download lands; Bindery's importer walks downloadDir and copies every recognised book file there into the audiobook library, dragging the unrelated siblings in.

Root cause: with Transmission a single-file torrent's downloadDir is the shared root itself, not a per-torrent subdirectory. filepath.Walk(downloadDir) then enumerates every sibling, and the importer treats them all as part of this torrent. qBittorrent and Deluge share the same shape when the user disables the default per-torrent subfolder option.

What changed

Per-client Files(ctx, downloadID) API

A new File{Name, Size} type lives in each downloader package, matching the existing per-package types pattern.

SABnzbd and NZBGet are intentionally not modified; both always land jobs inside a per-job destination folder (storage / DestDir), so a directory walk there cannot pick up sibling jobs. Documented in code comments and skipped in the test plan.

Threading through the importer

In internal/importer/scanner.go:

• tryImportInternal accepts a new explicitFiles []string parameter. When non-nil and non-empty, the importer uses it directly instead of filepath.Walk(downloadPath).
• tryImportTransmission / tryImportQbittorrent accept the file list and forward it.
• checkTransmissionDownloads / checkQbittorrentDownloads call the new transmissionFilesFor / qbittorrentFilesFor helpers right after deciding to import, then pass the result through.
• A new resolveTorrentFiles(client, savePath, []torrentFile) does the mapping from client-relative names to absolute Bindery-side paths, joining with savePath and running through remapDownloadClientPath so the existing per-client and global remap rules still apply uniformly.

Path-remap interaction

The new code reuses remapDownloadClientPath (the same helper that already rewrites torrent.DownloadDir for the parent dir). The flow per file is:

1. clientPath = filepath.Join(clientSavePath, file.Name) — the path the download client sees.
2. binderyPath = remapDownloadClientPath(client, clientPath) — the same per-client PathRemap rule that was already applied to the parent dir now rewrites the full file path consistently.
3. IsBookFile(binderyPath) filter, matching the legacy walk.

No new remap logic is introduced; the rules an operator sets in the UI keep working unchanged.

Fallback behaviour

When Files() returns an error or an empty list:

• transmissionFilesFor / qbittorrentFilesFor log a WARN (so an operator can see the fallback firing) and return nil.
• tryImportInternal falls back to the legacy filepath.Walk(downloadPath) discovery.

This preserves behaviour for older client protocol versions, transient RPC issues, and torrents that have not yet flushed metadata to disk (Transmission at 0% returns an empty files array).

Audiobook flow

The audiobook branch previously moved the entire downloadPath, which is the dangerous behaviour for #903's shape. With the file list:

• Single-file torrent: source is the file itself, placed inside destDir via the existing not-a-directory branch.
• Multi-file torrent whose files share a strict subdirectory under downloadPath: source is that subdirectory (so cover art and cue sheets within the torrent's folder still ride along).
• Multi-file torrent whose files sit at the shared root: falls back to per-file placement into destDir. Cover art outside the file list is left behind (the safer tradeoff vs. catching unrelated siblings).

Safety guards

File names containing an absolute path or any .. path segment are rejected with a WARN. Windows backslashes are normalised at the qBit API boundary so the segment check works uniformly.

Test plan

Unit tests for the downloader Files() APIs

• TestClient_Files_ReturnsRelativeNames (Transmission / qBittorrent / Deluge): canned RPC response → asserts []File shape matches.
• TestClient_Files_SingleFileTorrent (Transmission / qBittorrent / Deluge): the Bug Report: Single-file torrent import scans entire download directory #903 shape, basename-only Name.
• TestClient_Files_TorrentMissing (Transmission / qBittorrent / Deluge): unknown id surfaces an error so the importer can fall back.
• TestClient_Files_NoFilesYet (Transmission): empty array, nil error (metadata still resolving).
• TestClient_Files_WindowsBackslashesNormalized (qBittorrent): backslashes from a Windows qBit instance rewritten to forward slashes.
• TestClient_Files_StatusErrorReturnsError (Deluge): RPC-level fault surfaces as an error.

Importer-side tests

• TestImport_SingleFileTorrentSkipsSiblingFiles (end-to-end through checkTransmissionDownloads): shared download dir with the torrent's own .m4b plus two unrelated siblings; Transmission stub reports Files=[the-book.m4b]; asserts only the-book.m4b lands in the audiobook library and both siblings remain untouched.
• TestImport_FallsBackToDirWalkWhenFilesUnavailable: tryImportInternal with explicitFiles=nil (the post-RPC-failure shape) → asserts the dir walk runs and the book is imported.
• TestResolveTorrentFiles_BuildsAbsoluteBinderyPaths: covers PathRemap interaction and IsBookFile filtering.
• TestResolveTorrentFiles_RejectsMalformedNames: absolute paths and .. segments are dropped.
• TestResolveAudiobookSource_*: covers the single-file, common-subdir-accepted, and shared-root-falls-back-to-per-file branches.

Regression coverage

• Full go test ./... passes.
• go vet ./... clean.
• Existing Transmission and qBittorrent scanner tests pass unchanged; they now pass nil for explicitFiles and exercise the fallback walk.

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