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/SettingsView.vue

@@ -387,6 +387,14 @@ async function saveBriefingSettings() {
   }
 }
 
+async function toggleRss() {
+  try {
+    await store.updateSettings({ rss_enabled: store.rssEnabled ? "false" : "true" });
+  } catch {
+    toastStore.show("Failed to update RSS setting", "error");
+  }
+}
+
 async function addFeed() {
   if (!newFeedUrl.value.trim() || addingFeed.value) return;
   addingFeed.value = true;
@@ -2209,8 +2217,20 @@ function formatUserDate(iso: string): string {
         </p>
       </section>
 
-      <!-- RSS Feeds -->
+      <!-- RSS toggle -->
       <section class="settings-section full-width">
+        <h2>RSS / News</h2>
+        <div class="checkbox-field">
+          <label>
+            <input type="checkbox" :checked="store.rssEnabled" @change="toggleRss" />
+            Enable RSS feeds
+          </label>
+          <p class="field-hint">Subscribe to RSS/Atom feeds and include news in your briefings.</p>
+        </div>
+      </section>
+
+      <!-- RSS Feeds -->
+      <section v-if="store.rssEnabled" class="settings-section full-width">
         <div class="briefing-feeds-header">
           <div>
             <h2>RSS Feeds</h2>
@@ -2252,7 +2272,7 @@ function formatUserDate(iso: string): string {
       </section>
 
       <!-- News Preferences -->
-      <section class="settings-section full-width">
+      <section v-if="store.rssEnabled" class="settings-section full-width">
         <h2>News Preferences</h2>
         <p class="section-desc">
           Tell the briefing what topics you care about. Topics are matched against