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/configuration.md

@@ -17,6 +17,8 @@ except the session secret.
 | `VIBESONIC_SCAN_INTERVAL` | `1h` | How often to rescan enabled music folders, as a Go duration (`30m`, `6h`, ...). A startup scan always runs regardless of this value. |
 | `VIBESONIC_SESSION_SECRET` | *(none)* | Reserved for future use hardening session cookies beyond their database-backed random IDs. Not required to run, but recommended to set (e.g. `openssl rand -hex 32`) — VibeSonic logs a warning at startup if it's empty. |
 | `VIBESONIC_COOKIE_SECURE` | `true` | Whether session/CSRF cookies get the `Secure` attribute. Set to `false` only for local development over plain HTTP. |
+| `VIBESONIC_ENABLE_EXTERNAL_METADATA` | `false` | Enable optional MusicBrainz/Cover Art Archive lookups for songs/albums that already carry a locally-tagged MusicBrainz ID. VibeSonic works fully offline with this off; it's the only feature that makes outbound Internet requests. |
+| `VIBESONIC_PODCASTS_ALLOW_PRIVATE_NETWORKS` | `false` | Allow podcast feed/enclosure URLs to resolve to loopback/LAN/link-local addresses. Leave this off unless you intentionally host podcast feeds on a private network — see [`docs/security.md`](security.md#podcast-fetching-ssrf-protections). |
 
 ## External authentication