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>

src/fabledassistant/services/tools/_registry.py

@@ -84,6 +84,9 @@ async def _check_requires(user_id: int, requires: str) -> bool:
         return await is_caldav_configured(user_id)
     if requires == "searxng":
         return Config.searxng_enabled()
+    if requires == "rss":
+        from fabledassistant.services.settings import get_setting
+        return (await get_setting(user_id, "rss_enabled", "false")).lower() == "true"
     return True