feat: production-ready Linux & macOS support

- Add `linux` (dbus-python, PyGObject, python-xlib) and `macos`
  (pyobjc) extras to pyproject.toml with sys_platform markers; move
  cross-platform screen-brightness-control + monitorcontrol to base deps.
- build-dist-linux.sh: install `.[linux]`, pkg-config pre-flight for
  dbus-1/glib-2.0, emit a systemd unit with DBUS_SESSION_BUS_ADDRESS +
  XDG_RUNTIME_DIR + ReadWritePaths for ~/.config and ~/.cache so MPRIS
  works and audit-log / thumbnail writes aren't blocked by ProtectHome.
- New build-dist-macos.sh + per-user LaunchAgent installer producing
  MediaServer-vX.Y-macos-{arm64,x86_64}.tar.gz.
- Templated media-server.service updated to match the dist layout with
  proper session-bus env vars and a writable state-dir grant.
- install_linux.sh: drop dead requirements.txt path; install via
  `pip install ".[linux]"` and pre-create the writable state dirs.
- Cross-platform album artwork: abstract MediaController.get_album_art()
  with Linux (mpris:artUrl, file:// + http(s)://) and macOS (Spotify URL)
  impls; routes/media artwork endpoint now awaits the controller.
- LinuxMediaController connects to the session bus lazily — failure no
  longer crashes lifespan startup; MPRIS calls return idle until the bus
  is reachable. Logged once at INFO with a hint about
  `loginctl enable-linger`.
- Startup preflight on Linux warns if DBUS_SESSION_BUS_ADDRESS or
  XDG_RUNTIME_DIR is unset and informs the user when Wayland disables
  the foreground probe.
- /api/media/visualizer/status now reports a per-OS unavailable_reason.
- tray._confirm guarded against ctypes.windll on non-Windows.
- config.example.yaml: per-OS commented script examples; on_turn_off
  default is now a no-op echo (used to silently fail off Windows).
- README: replace stale `pip install -r requirements.txt` instructions
  with the new extras; add systemd lingering doc + troubleshooting
  section; add macOS LaunchAgent section.
- CI: new linux-smoke job (installs `.[linux]`, boots the server under
  dbus-run-session, asserts /api/health). Release workflow gains
  apt-deps step for the Linux build and a best-effort macOS build job.

media_server/services/linux_media.py

@@ -2,14 +2,23 @@
 
 import asyncio
 import logging
+import os
 import subprocess
+import threading
 from typing import Any, Optional
+from urllib.parse import unquote, urlparse
 
 from ..models import MediaState, MediaStatus
 from .media_controller import MediaController
 
 logger = logging.getLogger(__name__)
 
+# Cap remote artwork downloads so a hostile / huge Spotify image can't
+# blow up RAM. Real album art rarely exceeds ~2 MB; 8 MB is a comfortable
+# upper bound that also covers loss-less PNGs.
+_MAX_ART_BYTES = 8 * 1024 * 1024
+_ART_FETCH_TIMEOUT = 5.0  # seconds
+
 # Linux-specific imports
 try:
     import dbus
@@ -35,13 +44,54 @@ class LinuxMediaController(MediaController):
                 "Linux media control requires dbus-python package. "
                 "Install with: sudo apt-get install python3-dbus"
             )
-        DBusGMainLoop(set_as_default=True)
-        self._bus = dbus.SessionBus()
+        # The session-bus connection is deferred until first use. Connecting
+        # in __init__ raised during app startup whenever the user's session
+        # bus wasn't ready yet — common under systemd (service starts
+        # before logind set up /run/user/<uid>/bus), under SSH-without-X11,
+        # and in headless CI. Failing here killed the whole lifespan; now
+        # MPRIS calls return "idle" until the bus appears, and other
+        # endpoints (health, scripts, browser, …) keep working.
+        self._bus_lock = threading.Lock()
+        self._bus = None  # type: ignore[assignment]
+        self._bus_init_logged = False
+        # Cached art bytes keyed by the mpris:artUrl currently in flight.
+        # Lock guards the swap from the status thread vs the artwork handler.
+        self._art_lock = threading.Lock()
+        self._art_url: Optional[str] = None
+        self._art_bytes: Optional[bytes] = None
+
+    def _get_bus(self):
+        """Lazily connect to the session bus; returns None if unavailable."""
+        if self._bus is not None:
+            return self._bus
+        with self._bus_lock:
+            if self._bus is not None:
+                return self._bus
+            try:
+                DBusGMainLoop(set_as_default=True)
+                self._bus = dbus.SessionBus()
+                logger.info("Connected to D-Bus session bus")
+                return self._bus
+            except Exception as e:
+                # Log once at INFO to avoid log spam if every status poll fails.
+                if not self._bus_init_logged:
+                    logger.info(
+                        "D-Bus session bus not available (%s). "
+                        "MPRIS calls will return 'idle' until DBUS_SESSION_BUS_ADDRESS"
+                        " is set and the bus is reachable. Under systemd, ensure"
+                        " `loginctl enable-linger <user>` is set.",
+                        e,
+                    )
+                    self._bus_init_logged = True
+                return None
 
     def _get_active_player(self) -> Optional[str]:
         """Find an active MPRIS media player on the bus."""
+        bus = self._get_bus()
+        if bus is None:
+            return None
         try:
-            bus_names = self._bus.list_names()
+            bus_names = bus.list_names()
             mpris_players = [
                 name for name in bus_names if name.startswith(self.MPRIS_PREFIX)
             ]
@@ -181,7 +231,15 @@ class LinuxMediaController(MediaController):
             if artists:
                 status.artist = str(artists[0]) if isinstance(artists, list) else str(artists)
             status.album = str(metadata.get("xesam:album", "")) or None
-            status.album_art_url = str(metadata.get("mpris:artUrl", "")) or None
+            art_url = str(metadata.get("mpris:artUrl", "")) or None
+            status.album_art_url = art_url
+            # Invalidate cached bytes when the track changes. Real fetch
+            # happens lazily in get_album_art() so the status hot path
+            # never blocks on HTTP.
+            with self._art_lock:
+                if art_url != self._art_url:
+                    self._art_url = art_url
+                    self._art_bytes = None
             length = metadata.get("mpris:length", 0)
             if length:
                 status.duration = int(length) / 1_000_000
@@ -273,3 +331,61 @@ class LinuxMediaController(MediaController):
         except Exception as e:
             logger.error(f"Failed to open file {file_path}: {e}")
             return False
+
+    def _fetch_art_sync(self, url: str) -> Optional[bytes]:
+        """Resolve an ``mpris:artUrl`` to raw bytes (file://, http(s)://).
+
+        Other schemes (data:, ftp:, …) are rejected — we only support the
+        two cases real-world MPRIS providers use. The HTTP path is capped
+        at _MAX_ART_BYTES and the file path is read with a size guard so a
+        symlink to /dev/zero can't OOM the server.
+        """
+        try:
+            parsed = urlparse(url)
+        except ValueError:
+            return None
+
+        scheme = parsed.scheme.lower()
+        if scheme == "file":
+            path = unquote(parsed.path)
+            try:
+                size = os.stat(path).st_size
+                if size <= 0 or size > _MAX_ART_BYTES:
+                    return None
+                with open(path, "rb") as f:
+                    return f.read(_MAX_ART_BYTES)
+            except OSError as e:
+                logger.debug("Could not read local art %s: %s", path, e)
+                return None
+
+        if scheme in ("http", "https"):
+            import urllib.error
+            import urllib.request
+
+            req = urllib.request.Request(url, headers={"User-Agent": "media-server/0.x"})
+            try:
+                with urllib.request.urlopen(req, timeout=_ART_FETCH_TIMEOUT) as resp:
+                    # Cap reads to defend against unbounded responses.
+                    return resp.read(_MAX_ART_BYTES + 1)[:_MAX_ART_BYTES] or None
+            except (urllib.error.URLError, urllib.error.HTTPError, OSError) as e:
+                logger.debug("Could not fetch remote art %s: %s", url, e)
+                return None
+
+        logger.debug("Unsupported art URL scheme: %s", scheme)
+        return None
+
+    async def get_album_art(self) -> Optional[bytes]:
+        """Return cached MPRIS art, fetching on first access per track."""
+        with self._art_lock:
+            url = self._art_url
+            cached = self._art_bytes
+        if cached is not None:
+            return cached
+        if not url:
+            return None
+        data = await asyncio.to_thread(self._fetch_art_sync, url)
+        # Store even on None so we don't re-hammer a 404 every second.
+        with self._art_lock:
+            if url == self._art_url:
+                self._art_bytes = data
+        return data