feat(schedule): per-plugin schedules and hide-while-live

Users asked to turn off news and stocks on gameday so only scores show.
Each plugin can now have its own schedule (global or per-day window in
which it may show) plus hide_when_live, which hides it whenever any
plugin reports a live game. A hidden plugin is skipped in the rotation,
the live takeover and the Vegas ticker; it stays loaded, and on-demand
still shows it.

- src/plugin_schedule.py: the window rule, now shared with the display
  schedule (_check_schedule calls schedule_state)
- core config key plugin_schedules; GET/POST /api/v3/config/plugin-schedules
- Schedule page: Plugin Schedules card, one picker per enabled plugin

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

web_interface/static/v3/js/widgets/schedule-picker.js

@@ -188,7 +188,7 @@
                                    class="h-4 w-4 text-blue-600 focus:ring-blue-500 border-gray-300 rounded">
                             <span class="ml-2 text-sm font-medium text-gray-900">Enable Schedule</span>
                         </label>
-                        <p class="mt-1 text-sm text-gray-600">When enabled, the display will only operate during specified hours.</p>
+                        <p class="mt-1 text-sm text-gray-600">${window.LEDEscape.html(xOptions.enableDescription || 'When enabled, the display will only operate during specified hours.')}</p>
                     </div>
                 `;
             }