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>

test/fixtures/api_v3_url_map.json

@@ -159,6 +159,23 @@
       "POST"
     ]
   ],
+  [
+    "/api/v3/config/plugin-schedules",
+    "api_v3.get_plugin_schedules",
+    [
+      "GET",
+      "HEAD",
+      "OPTIONS"
+    ]
+  ],
+  [
+    "/api/v3/config/plugin-schedules",
+    "api_v3.save_plugin_schedule",
+    [
+      "OPTIONS",
+      "POST"
+    ]
+  ],
   [
     "/api/v3/config/raw/main",
     "api_v3.save_raw_main_config",