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/task.go

@@ -155,6 +155,13 @@ func (t *Task) GetStreamURL() string {
 	return t.StreamURL
 }
 
+// SafeFilePath returns the task's final file path thread-safely.
+func (t *Task) SafeFilePath() string {
+	t.mu.RLock()
+	defer t.mu.RUnlock()
+	return t.FilePath
+}
+
 // UpdateProgress updates download metrics thread-safely.
 func (t *Task) UpdateProgress(p Progress) {
 	t.mu.Lock()