Add a setting to hide the quarantine from Plex

Settings -> Media servers -> "Hide quarantine from Plex" writes a
.plexignore into every pair's quarantine folder (.playlink-trash by
default), with patterns for every folder depth quarantined files live
at. Quarantine folders opened later get the file automatically.

The file carries a marker line: turning the option off removes only
playlink's own file, and an existing .plexignore written by someone else
is never overwritten or deleted (reported instead). Writes go through a
temporary file and a rename.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

README.md

@@ -152,6 +152,10 @@ Run a scan, check the Review tab, and turn off dry run once the planned actions
 | `PLAYLINK_FFPROBE` | `ffprobe` | Path to ffprobe. Without it, quality comparison is disabled but everything else works. |
 | `PLAYLINK_LOG_LEVEL` | `info` | `debug` for verbose logs. |
 
+**Hide quarantine from Plex** (Settings → Media servers) writes a `.plexignore` into each pair's quarantine folder so Plex does not
+pick up quarantined songs; new quarantine folders get it automatically. Turning it off removes the file again, but only if playlink
+created it.
+
 Per-pair settings (in the UI) are file extensions, auto-link, approximate matching and its threshold, scan interval, quarantine folder,
 retention and settle time.