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/engine/manager.go

@@ -33,6 +33,12 @@ type Manager struct {
 	// Used by the daemon to trigger an immediate sync.
 	OnTaskDone func()
 
+	// OnFinalized is called after a task successfully transitions to
+	// completed (after verify + organize + optional upgrade replace).
+	// Used by the daemon to trigger media-server library refreshes.
+	// Not invoked on failure.
+	OnFinalized func(task *Task)
+
 	// recentlyFinished holds tasks that completed/failed since the last sync read.
 	// The sync goroutine reads and clears this to include final states in the next sync.
 	recentMu       sync.Mutex
@@ -444,6 +450,9 @@ func (m *Manager) finalize(ctx context.Context, task *Task, result *Result) {
 	if m.cfg.Notifications {
 		desktopNotify("Download complete", task.Title)
 	}
+	if m.OnFinalized != nil {
+		m.OnFinalized(task)
+	}
 	m.recordFinished(task.ToStatusUpdate())
 	m.reporter.ReportFinal(ctx, task)
 }