Fix handling of single media file updates

[mikiher]

Resolves #2282.

After the fix, relevant logs lines:

[2023-11-04 11:03:26] DEBUG: [Watcher] File Added /torrents/2BR02B - librivox.m4b (Watcher.js:151)
[2023-11-04 11:03:26] DEBUG: [Watcher] Modified file in library "Torrents" and folder "7574feec-eebc-4858-a01e-7613f1eac121" with relPath "/2BR02B - librivox.m4b" (Watcher.js:256)
[2023-11-04 11:03:26] DEBUG: [Watcher] Waiting to add file at "/torrents/2BR02B - librivox.m4b". mtimeMs=1696863726581.3113 lastMTimeMs=0 (loop 0) (Watcher.js:209)
[2023-11-04 11:03:27] DEBUG: [Watcher] File finished adding at "/torrents/2BR02B - librivox.m4b" (Watcher.js:205)
[2023-11-04 11:03:30] DEBUG: [Scanner] Scanning file update groups in folder "7574feec-eebc-4858-a01e-7613f1eac121" of library "Torrents" (LibraryScanner.js:459)
[2023-11-04 11:03:30] DEBUG: [Scanner] scanFolderUpdates fileUpdateGroup { '2BR02B - librivox.m4b': '2BR02B - librivox.m4b' } (LibraryScanner.js:460)
[2023-11-04 11:03:31] DEBUG: [LibraryScanner] Folder update group must be a new item "2BR02B - librivox.m4b" in library "Torrents" (LibraryScanner.js:582)
[2023-11-04 11:03:31] DEBUG: [AudioFileScanner] Smart track order for "2BR02B - librivox.m4b" using track key trackNumFromMeta (AudioFileScanner.js:94)
[2023-11-04 11:03:31] DEBUG: [Scan] "2BR02B - librivox.m4b": "2BR02B - librivox" Getting metadata with precedence [folderStructure, audioMetatags, txtFiles, opfFile, absMetadata] (ScanLogger.js:65)
[2023-11-04 11:03:31] DEBUG: [Scan] "2BR02B - librivox.m4b": setChapters: Using embedded chapters in first audio file /torrents/2BR02B - librivox.m4b (ScanLogger.js:65)
[2023-11-04 11:03:31] DEBUG: [Scan] "2BR02B - librivox.m4b": Success saving abmetadata to "/workspaces/audiobookshelf/metadata/items/72fb15fe-b773-48c4-a40a-e6f1cb4a97f9/metadata.json" (ScanLogger.js:65)
[2023-11-04 11:03:31] DEBUG: [Scan] "2BR02B - librivox.m4b": Created new library item "2BR02B - librivox.m4b" (ScanLogger.js:65)
[2023-11-04 11:03:31] DEBUG: [LibraryScanner] Folder scan results { '2BR02B - librivox.m4b': 1 } (LibraryScanner.js:386)

[advplyr]

Works well. Thanks!