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_test.go

@@ -79,7 +79,7 @@ func TestPlexTokenFromPrefs(t *testing.T) {
 <Preferences PlexOnlineToken="my-secret-token" OldestPreviousVersion="1.0"/>`
 		os.WriteFile(prefsPath, []byte(xml), 0o644)
 
-		token := plexTokenFromPrefs(prefsPath)
+		token := PlexTokenFromPrefs(prefsPath)
 		if token != "my-secret-token" {
 			t.Errorf("token = %q, want my-secret-token", token)
 		}
@@ -91,14 +91,14 @@ func TestPlexTokenFromPrefs(t *testing.T) {
 		xml := `<?xml version="1.0"?><Preferences/>`
 		os.WriteFile(prefsPath, []byte(xml), 0o644)
 
-		token := plexTokenFromPrefs(prefsPath)
+		token := PlexTokenFromPrefs(prefsPath)
 		if token != "" {
 			t.Errorf("token = %q, want empty", token)
 		}
 	})
 
 	t.Run("file not found", func(t *testing.T) {
-		token := plexTokenFromPrefs("/nonexistent/Preferences.xml")
+		token := PlexTokenFromPrefs("/nonexistent/Preferences.xml")
 		if token != "" {
 			t.Errorf("token = %q, want empty", token)
 		}
@@ -109,7 +109,7 @@ func TestPlexTokenFromPrefs(t *testing.T) {
 		prefsPath := filepath.Join(dir, "Preferences.xml")
 		os.WriteFile(prefsPath, []byte("not xml at all"), 0o644)
 
-		token := plexTokenFromPrefs(prefsPath)
+		token := PlexTokenFromPrefs(prefsPath)
 		if token != "" {
 			t.Errorf("token = %q, want empty", token)
 		}