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/views/BriefingView.vue

@@ -2,6 +2,7 @@
 import { ref, computed, onMounted, onUnmounted, watch, nextTick } from 'vue'
 import { useBackgroundRefresh } from '@/composables/useBackgroundRefresh'
 import { useChatStore } from '@/stores/chat'
+import { useSettingsStore } from '@/stores/settings'
 import ChatPanel from '@/components/ChatPanel.vue'
 import WeatherCard from '@/components/WeatherCard.vue'
 import BriefingSetupWizard from '@/components/BriefingSetupWizard.vue'
@@ -33,6 +34,7 @@ interface WeatherData {
 }
 
 const chatStore = useChatStore()
+const settingsStore = useSettingsStore()
 
 // Setup wizard
 const showWizard = ref(false)
@@ -293,7 +295,7 @@ onMounted(async () => {
         </div>
 
         <!-- News section (scrollable) -->
-        <div class="news-section">
+        <div v-if="settingsStore.rssEnabled" class="news-section">
         <div class="panel-label-row">
           <div class="panel-label">Today's News</div>
           <span v-if="newsItems.length" class="news-count">{{ newsItems.length }} items</span>