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/cmd/root.go

@@ -117,6 +117,9 @@ Source:         https://github.com/torrentclaw/unarr`,
 	scanCmd := newScanCmd()
 	scanCmd.GroupID = "search"
 
+	mediaserverCmd := newMediaserverCmd()
+	mediaserverCmd.GroupID = "start"
+
 	rootCmd.AddCommand(
 		// Getting Started
 		initCmd,
@@ -146,6 +149,7 @@ Source:         https://github.com/torrentclaw/unarr`,
 		completionCmd,
 		// Library
 		scanCmd,
+		mediaserverCmd,
 		// Alias: upgrade → self-update
 		newUpgradeCmd(),
 	)