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/init.go

@@ -296,6 +296,34 @@ func runInit(apiURLOverride string) error {
 		}
 	}
 
+	// ── Plex / Jellyfin / Emby refresh hook ────────────────────────
+	// Offer to wire library refreshes if a media server was detected and
+	// none are configured yet. Skipping here is fine — the user can run
+	// `unarr mediaserver setup` later.
+	if len(detected.Servers) > 0 && len(cfg.MediaServers) == 0 {
+		fmt.Println()
+		var configureMS bool
+		err = huh.NewForm(
+			huh.NewGroup(
+				huh.NewConfirm().
+					Title(fmt.Sprintf("Auto-refresh %s on every download?", detected.Servers[0].Name)).
+					Description("New downloads appear on Roku/Apple TV/etc. within seconds, instead of waiting for the next periodic library scan").
+					Affirmative("Yes, configure").
+					Negative("Skip (do it later with 'unarr mediaserver setup')").
+					Value(&configureMS),
+			),
+		).Run()
+		if err == nil && configureMS {
+			fmt.Println()
+			if err := runMediaserverSetup(); err != nil {
+				color.New(color.FgYellow).Printf("  Media server setup failed: %s\n", err)
+			} else {
+				// runMediaserverSetup already saved + updated appCfg.
+				cfg = appCfg
+			}
+		}
+	}
+
 	// ── Debrid auto-detection from *arr ─────────────────────────────
 
 	if resp.User.IsPro {