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

docs/external-auth.md

@@ -59,6 +59,10 @@ password on VibeSonic's own `/login` page even when external auth is
 enabled, unless their account has no password hash set (auto-created
 external users start this way).
 
-In later phases, `/rest/*` (the native Subsonic API) will authenticate
-exclusively via Subsonic's own token/API-key scheme and must never be
-affected by external auth's browser-cookie flow.
+`/rest/*` (the Subsonic/OpenSubsonic API) never consults external auth
+at all: it authenticates exclusively via Subsonic's own token/salt,
+password, or `apiKey` scheme (see `internal/subsonic/auth.go`), so real
+Subsonic/OpenSubsonic clients keep working unmodified even when external
+auth fronts the browser UI. This is also why a reverse proxy enforcing
+SSO in front of VibeSonic should let `/rest/*` bypass that SSO layer —
+see `docs/deployment.md` for example proxy configuration.