feat(mediaserver): Plex/Jellyfin/Emby auto-refresh + .strm instant mode

Sprint 1 — Auto-refresh after download:
- New [[mediaserver]] TOML section with kind/url/token/sections
- mediaserver.Refresh() fans out to Plex (partial via section ID auto-mapping
  from file path prefix) and Jellyfin/Emby (full library scan)
- Manager.OnFinalized callback wired in daemon to trigger refresh after
  organize() completes — keeps engine package free of mediaserver dep
- New unarr mediaserver {setup,list,remove,test} commands
- unarr init wizard offers to configure refresh when a server is detected

Sprint 2 — .strm instant mode (cloud + agent):
- Mode strm-to-library handled in daemon dispatch: writes a one-line .strm
  file pointing to the cloud-resolved debrid HTTPS URL, then triggers refresh
- engine.WriteStrm + StrmDestForTask mirror organize()'s naming so Plex/Jellyfin
  see the expected folder structure (Movies/Title (Year)/, TV Shows/Show/Season XX/)
- Atomic write (temp + rename) so partial files never get indexed
- Reports completed/failed status to the cloud via existing agent client

internal/mediaserver/detect.go

@@ -87,6 +87,17 @@ func Detect() DetectedPaths {
 
 // ── Plex ────────────────────────────────────────────────────────────
 
+// LocalPlexToken returns the Plex auth token from the local Plex config
+// directory, if Plex Media Server is installed on this host. Returns ""
+// when Plex isn't installed or the token can't be read.
+func LocalPlexToken() string {
+	dir := plexConfigDir()
+	if dir == "" {
+		return ""
+	}
+	return PlexTokenFromPrefs(filepath.Join(dir, "Preferences.xml"))
+}
+
 func plexLibraryPaths() []string {
 	configDir := plexConfigDir()
 	if configDir == "" {
@@ -95,7 +106,7 @@ func plexLibraryPaths() []string {
 
 	// Read token from Preferences.xml
 	prefsPath := filepath.Join(configDir, "Preferences.xml")
-	token := plexTokenFromPrefs(prefsPath)
+	token := PlexTokenFromPrefs(prefsPath)
 	if token == "" {
 		return nil
 	}
@@ -154,7 +165,10 @@ type plexPrefs struct {
 	PlexOnlineToken string   `xml:"PlexOnlineToken,attr"`
 }
 
-func plexTokenFromPrefs(path string) string {
+// PlexTokenFromPrefs reads the Plex auth token from a Preferences.xml file.
+// Returns "" if the file can't be read or parsed. Used by the setup wizard
+// when configuring a Plex server running on the same host as unarr.
+func PlexTokenFromPrefs(path string) string {
 	data, err := os.ReadFile(path)
 	if err != nil {
 		return ""