fix: production-readiness hardening — security, perf, a11y, observability

Security
- Default scripts_management, callbacks_management, links_management, and
  media_folders_management to False so a leaked token cannot escalate to RCE
  through admin CRUD endpoints.
- TokenSpec + scope hierarchy (read | control | admin); legacy bare-string
  api_tokens entries promote to admin for back-compat. Management endpoints
  now require admin scope.
- WebSocket subprotocol auth (Sec-WebSocket-Protocol: media-server.token.<T>)
  preferred over ?token= query so the token no longer lands in URL/history/
  Referer; query fallback retained for HA integration back-compat.
- Origin allow-list check on the WS endpoint (CSWSH defence).
- In-process token-bucket rate limiter: 5/min for failed auths,
  10/min for /api/scripts/execute and /api/callbacks/execute.
- shell=False subprocess path (shlex.split) + per-parameter regex `pattern`
  in ScriptParameterConfig to harden shell=true scripts against parameter
  injection (Windows cmd.exe env-var expansion).
- CSP gains form-action, worker-src, manifest-src directives.
- Refuse cors_origins=["*"] at startup; strip token=... from uvicorn access
  logs; validate Gitea release tag against strict SemVer regex.
- noopener noreferrer + no-referrer referrerpolicy on every outbound link.
- icacls hardening of config.yaml on Windows (current user + SYSTEM +
  Administrators only); 0600 still enforced on POSIX.
- WS volume handler clamps input and never drops the socket on bad messages.

Performance
- Album-art read in windows_media gated by track key — was decoding the
  WinRT thumbnail twice per second regardless of track changes.
- /api/media/artwork returns content-derived ETag + Cache-Control so the
  browser sends If-None-Match and gets 304s on track repeats.
- Foreground-service ctypes argtypes hoisted to one-time module init
  (was re-declaring ~14 prototypes per probe).
- display_service _static_cache keyed by (edid_hash, ...) tuple with
  eviction of disappeared monitors — fixes stale capabilities on hot-plug
  swaps where the new topology has the same monitor count.
- Visualizer rAF loop paused on document.hidden, resumed on visible.

Reliability / bug fixes
- Lifespan rewritten as try/yield/finally so a partial-startup failure
  cannot orphan background tasks or executors.
- _run_callback in routes/media.py keeps a strong task ref (GC-safe) and
  uses the dedicated callback executor instead of the default pool.
- macos_media.set_volume() no longer always returns True.
- TrayManager._restart_requested initialised in __init__; set before
  signalling exit so the main thread observes it correctly.
- Missing static_dir now logs a WARNING instead of silent UI disable.

UX / accessibility / PWA
- manifest.json theme_color and background_color match the Studio Reference
  base (#0E0D0B); added id and scope for PWA installability.
- ARIA on mini-player icon buttons; inner SVGs marked aria-hidden.
- OS mediaSession API wired so headset / lockscreen / Bluetooth buttons
  drive play/pause/next/prev/seek and show track metadata + artwork.

Observability
- X-Request-ID middleware (accept upstream id if it matches a safe regex,
  otherwise UUID4); request_id_var added to ContextVars and included in
  every log line alongside the token label.
- Audit log (append-only JSONL) for every script + callback execution,
  including the on_play/on_pause/etc. event callbacks. Background-thread
  writer; queue capped; flushed in lifespan teardown.

Deployment
- proxy_headers + forwarded_allow_ips plumbed through Settings →
  uvicorn.Config for reverse-proxy installs.
- HTTPS support via ssl_certfile + ssl_keyfile (+ optional password);
  startup refuses to launch with only one of the pair set.
- Thumbnail cache moved from project-root .cache to
  %LOCALAPPDATA%/media-server/cache (Windows) and
  $XDG_CACHE_HOME/media-server/thumbnails (POSIX).

Tests
- 35 new tests across auth scopes, rate limiter, browser path traversal
  (../ NUL UNC absolute), script-param validation incl. regex, Gitea tag
  whitelist, config atomic write + POSIX perms. 47 passed / 4 skipped.

media_server/static/js/player.js

@@ -10,6 +10,7 @@ import {
     lastStatus, setLastStatus, currentPlayState, setCurrentPlayState,
     POSITION_INTERPOLATION_MS, seek, notifyRemoteVolume,
     getAuthHeaders, hasCredentials,
+    togglePlayPause, nextTrack, previousTrack,
 } from './core.js';
 import { updateBackgroundColors } from './background.js';
 import { loadDisplayMonitors } from './links.js';
@@ -381,11 +382,85 @@ function buildVisualizerGradient() {
 
 function startVisualizerRender() {
     if (visualizerAnimFrame) return;
+    // Don't even queue a frame while the tab is hidden — rAF still fires on
+    // hidden tabs (throttled but not paused) and would burn CPU + battery
+    // smoothing into bars no one can see. We resume on `visibilitychange`.
+    if (typeof document !== 'undefined' && document.hidden) return;
     // Cache editorial spectrum bar refs once per start.
     cacheEditorialSpectrumBars();
     renderVisualizerFrame();
 }
 
+// ─── OS Media Session integration ─────────────────────────────
+// Hooks the page into the system's media session so headset / lockscreen /
+// Bluetooth play-pause-skip buttons drive the active track. Action handlers
+// are set once and never re-registered; only the metadata + playback state
+// flip when a track changes.
+let _mediaSessionInitialised = false;
+let _lastMediaSessionKey = '';
+function syncMediaSession(status) {
+    if (typeof navigator === 'undefined' || !('mediaSession' in navigator)) return;
+    const session = navigator.mediaSession;
+
+    if (!_mediaSessionInitialised) {
+        const setHandler = (name, fn) => {
+            try { session.setActionHandler(name, fn); } catch { /* unsupported action */ }
+        };
+        setHandler('play', () => togglePlayPause());
+        setHandler('pause', () => togglePlayPause());
+        setHandler('nexttrack', () => nextTrack());
+        setHandler('previoustrack', () => previousTrack());
+        setHandler('seekto', (ev) => { if (ev && typeof ev.seekTime === 'number') seek(ev.seekTime); });
+        _mediaSessionInitialised = true;
+    }
+
+    // Track-identity key — re-build metadata only when title/artist/album change.
+    const artworkSrc = status && status.album_art_url ? '/api/media/artwork' : '';
+    const key = `${status.title || ''}|${status.artist || ''}|${status.album || ''}|${artworkSrc}`;
+    if (key !== _lastMediaSessionKey) {
+        _lastMediaSessionKey = key;
+        try {
+            session.metadata = new MediaMetadata({
+                title: status.title || '',
+                artist: status.artist || '',
+                album: status.album || '',
+                artwork: artworkSrc ? [{ src: artworkSrc, sizes: '512x512', type: 'image/*' }] : [],
+            });
+        } catch { /* MediaMetadata unsupported on very old browsers */ }
+    }
+
+    session.playbackState =
+        status.state === 'playing' ? 'playing'
+        : status.state === 'paused' ? 'paused'
+        : 'none';
+
+    if (typeof session.setPositionState === 'function'
+        && status.duration && status.duration > 0
+        && typeof status.position === 'number') {
+        try {
+            session.setPositionState({
+                duration: status.duration,
+                position: Math.min(status.position, status.duration),
+                playbackRate: 1.0,
+            });
+        } catch { /* invalid range — ignore */ }
+    }
+}
+
+// Pause / resume the visualizer with tab visibility. Idempotent: called once
+// at module init below, no-op if no listener support.
+if (typeof document !== 'undefined' && document.addEventListener) {
+    document.addEventListener('visibilitychange', () => {
+        if (document.hidden) {
+            stopVisualizerRender();
+        } else if (frequencyData) {
+            // Only restart if a payload is live (otherwise startVisualizerRender
+            // would queue a no-op rAF chain forever waiting for one).
+            startVisualizerRender();
+        }
+    });
+}
+
 export function stopVisualizerRender() {
     if (visualizerAnimFrame) {
         cancelAnimationFrame(visualizerAnimFrame);
@@ -903,6 +978,11 @@ export function updateUI(status) {
 
     updateMuteIcon(status.muted);
 
+    // Wire the OS Media Session so headset buttons, lockscreen controls, and
+    // Bluetooth remotes drive the active media (not the WebUI tab). Cheap and
+    // idempotent — re-running setActionHandler with the same fn is a no-op.
+    syncMediaSession(status);
+
     const src = resolveMediaSource(status.source);
     dom.source.textContent = src ? src.name : t('player.unknown_source');
     dom.sourceIcon.innerHTML = src?.icon || '';