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>

internal/engine/engine.go

@@ -229,6 +229,13 @@ func (e *Engine) open(p store.Pair) (*libs, error) {
 	if err := os.MkdirAll(q, 0o775); err != nil {
 		return nil, fmt.Errorf("quarantine directory: %w", err)
 	}
+	// Keep the quarantine hidden from Plex when enabled, including quarantine
+	// folders created after the setting was turned on. Never fatal.
+	if e.plexIgnoreEnabled() {
+		if err := syncPlexIgnore(q, true); err != nil && !errors.Is(err, ErrForeignIgnore) {
+			e.log.Warn(".plexignore not written", "err", plexIgnoreError(q, err))
+		}
+	}
 	if rq, err := filepath.EvalSymlinks(q); err != nil || (rq != q && !e.withinAllowed(rq)) {
 		return nil, errors.New("quarantine directory resolves outside the allowed roots")
 	}