feat(05-01): add cue scheduling loop and preview_update broadcast to tick handler

- LOOKAHEAD = 0.060 (60ms window per D-02)
- Scheduler iterates show.tracks/cues on each tick, gated on is_playing and show
- Cues within lookahead window fire UDP via encode_animation_cmd + UDPSender.send
- Past cues (>100ms behind) silently marked fired without UDP dispatch
- active_cue tracking per track enables preview state for already-fired blocks
- preview_update messages broadcast to all clients on each tick (D-07)
- Clear preview_update (animation: null) sent when no active cue on a device

lightsync/api/ws.py

@@ -129,6 +129,68 @@ async def websocket_endpoint(websocket: WebSocket):
                                 "tempo": tempo,
                             }))
 
+                # ── Cue scheduler (Phase 5 — D-01, D-02) ──────────────────
+                if is_playing and show is not None:
+                    LOOKAHEAD = 0.060  # 60ms look-ahead window (D-02)
+                    udp = websocket.app.state.udp_sender
+                    preview_updates = []  # batch preview messages
+
+                    for track in show.tracks:
+                        device = next(
+                            (d for d in show.devices if str(d.id) == str(track.device_id)),
+                            None,
+                        )
+                        if device is None:
+                            continue
+
+                        active_cue = None  # track which cue is active for preview
+
+                        for cue in track.cues:
+                            cue_id = str(cue.id)
+                            if cue_id in fired_ids:
+                                # Check if this already-fired cue is still active (for preview)
+                                if cue.timestamp <= position < cue.timestamp + cue.duration:
+                                    active_cue = cue
+                                continue
+                            if cue.timestamp > position + LOOKAHEAD:
+                                continue  # not due yet
+                            if cue.timestamp < position - 0.1:
+                                # Too far in the past — mark fired without sending
+                                fired_ids.add(cue_id)
+                                continue
+
+                            # Fire this cue (D-01)
+                            fired_ids.add(cue_id)
+                            active_cue = cue
+                            if cue.animation:
+                                try:
+                                    payload = encode_animation_cmd(cue.animation, cue.params)
+                                    udp.send(payload, device.ip, device.port)
+                                except (ValueError, KeyError):
+                                    logger.warning("[ws] unknown animation %r", cue.animation)
+
+                        # Build preview update for this device (D-07)
+                        if active_cue and active_cue.animation:
+                            color = active_cue.params.get("color", [0, 255, 180])
+                            preview_updates.append({
+                                "type": "preview_update",
+                                "device_id": str(track.device_id),
+                                "animation": active_cue.animation,
+                                "color": color,
+                            })
+                        else:
+                            # No active cue — send clear (D-05: turns dark)
+                            preview_updates.append({
+                                "type": "preview_update",
+                                "device_id": str(track.device_id),
+                                "animation": None,
+                                "color": None,
+                            })
+
+                    # Broadcast all preview updates
+                    for pu in preview_updates:
+                        await manager.broadcast(pu)
+
             elif msg_type == "play":
                 logger.info("[ws] play event at position=%.3f", float(msg.get("position", 0)))
                 last_beat_reported = None  # reset beat tracking on play