feat: add rss_enabled user setting to toggle RSS functionality

RSS is now off by default. When disabled:
- Scheduler skips RSS feed sync during compilation slot
- Briefing pipeline skips RSS item gathering
- RSS LLM tools (get_rss_items, add_rss_feed) are hidden
- API routes return empty results for feeds/news
- Frontend hides News nav link, RSS Feeds and News Preferences in settings
- Briefing view hides news sidebar section

Toggle in Settings > Briefing > RSS / News.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/stores/settings.ts

@@ -16,6 +16,10 @@ export const useSettingsStore = defineStore("settings", () => {
     () => settings.value.default_model || ""
   );
 
+  const rssEnabled = computed(
+    () => settings.value.rss_enabled === "true"
+  );
+
   // Voice status — checked once on login, refreshable from Settings
   const voiceEnabled = ref(false);
   const voiceSttReady = ref(false);
@@ -62,6 +66,7 @@ export const useSettingsStore = defineStore("settings", () => {
     loading,
     assistantName,
     defaultModel,
+    rssEnabled,
     voiceEnabled,
     voiceSttReady,
     voiceTtsReady,