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/components/AppHeader.vue

@@ -5,6 +5,7 @@ import { useTheme } from "@/composables/useTheme";
 import { useShortcuts } from "@/composables/useShortcuts";
 import { useAuthStore } from "@/stores/auth";
 import { useChatStore } from "@/stores/chat";
+import { useSettingsStore } from "@/stores/settings";
 import AppLogo from "@/components/AppLogo.vue";
 import NotificationBell from "@/components/NotificationBell.vue";
 
@@ -12,6 +13,7 @@ const { theme, toggleTheme } = useTheme();
 const { toggleShortcuts } = useShortcuts();
 const authStore = useAuthStore();
 const chatStore = useChatStore();
+const settingsStore = useSettingsStore();
 const router = useRouter();
 const route = useRoute();
 
@@ -78,7 +80,7 @@ router.afterEach(() => {
           <router-link to="/chat" :class="['nav-link', { 'router-link-active': isChatActive }]">Chat</router-link>
           <router-link to="/briefing" class="nav-link">Briefing</router-link>
           <router-link to="/calendar" class="nav-link">Calendar</router-link>
-          <router-link to="/news" class="nav-link">News</router-link>
+          <router-link v-if="settingsStore.rssEnabled" to="/news" class="nav-link">News</router-link>
           <router-link to="/projects" class="nav-link">Projects</router-link>
         </div>
       </div>
@@ -128,7 +130,7 @@ router.afterEach(() => {
       <router-link to="/briefing" class="nav-link">Briefing</router-link>
       <router-link to="/calendar" class="nav-link">Calendar</router-link>
       <router-link to="/projects" class="nav-link">Projects</router-link>
-      <router-link to="/news" class="nav-link">News</router-link>
+      <router-link v-if="settingsStore.rssEnabled" to="/news" class="nav-link">News</router-link>
       <router-link to="/shared" class="nav-link">Shared</router-link>
       <div class="mobile-divider"></div>
       <router-link to="/settings" class="nav-link">Settings</router-link>