Complete Phase 4: security hardening, player/playlist/admin UI, docs

Security:
- New internal/netguard package: podcast feed/enclosure fetching now
  refuses to dial non-public IPs (loopback/RFC1918/link-local/cloud
  metadata/CGNAT), validated per-connection including across redirects,
  closing an SSRF gap where a malicious or compromised feed URL could
  probe the server's internal network. Configurable via
  VIBESONIC_PODCASTS_ALLOW_PRIVATE_NETWORKS for intentional LAN feeds.
- Podcast episode downloads capped at 4GB (was unbounded, disk-exhaustion risk)
- Cover art decoding now rejects images above a 40-megapixel budget
  before full decode (decompression-bomb guard)
- HTTP server sets ReadHeaderTimeout (slowloris hardening)
- Admin folder-scan redirect now validated as same-site instead of
  trusting the raw Referer header (open-redirect fix)
- Legacy Subsonic token/salt auth now uses a constant-time comparison

Web UI completion:
- Persistent player: shuffle and repeat (off/all/one) controls, wired
  into queue advance/prev logic and persisted like other player state
- Admin user-permissions form: adminRole toggle (with a guard against
  demoting the last remaining admin) and per-user music-folder access
  checkboxes, backed by existing schema/enforcement that was previously
  unreachable from the UI
- Playlist M3U8 import/export

Docs: added docs/client-compatibility.md (honest tracking template —
no client testing has been performed), docs/deployment.md,
docs/security.md, docs/backup.md; refreshed README and cross-links;
added a footnote in docs/subsonic-compatibility.md distinguishing
spec-standard from VibeSonic-original OpenSubsonic extensions.

Also added Go fuzz tests for the external-auth CIDR/IP parser and the
LRC lyrics parser.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_014r98AtcBbevpscWLsfwVvv

internal/app/app.go

@@ -73,7 +73,7 @@ func Open(cfg config.Config, log *slog.Logger) (*App, error) {
 		return nil, fmt.Errorf("deriving auth encryption key: %w", err)
 	}
 
-	podcastMgr := podcasts.NewManager(db, cfg.DataDir, cfg.FFprobePath, log)
+	podcastMgr := podcasts.NewManager(db, cfg.DataDir, cfg.FFprobePath, log, cfg.PodcastsAllowPrivateNetworks)
 	if err := podcastMgr.RecoverInterruptedDownloads(); err != nil {
 		log.Warn("podcast download recovery failed", "err", err)
 	}
@@ -165,8 +165,9 @@ func (a *App) Run(ctx context.Context) error {
 	mux.Handle("/", a.Web.NewRouter())
 
 	srv := &http.Server{
-		Addr:    a.Cfg.ListenAddr,
-		Handler: mux,
+		Addr:              a.Cfg.ListenAddr,
+		Handler:           mux,
+		ReadHeaderTimeout: 15 * time.Second,
 	}
 
 	errCh := make(chan error, 1)