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/rss.py

@@ -18,6 +18,7 @@ logger = logging.getLogger(__name__)
     },
     read_only=True,
     briefing=True,
+    requires="rss",
 )
 async def get_rss_items_tool(*, user_id, arguments, **_ctx):
     from fabledassistant.services.rss import get_recent_items
@@ -35,6 +36,7 @@ async def get_rss_items_tool(*, user_id, arguments, **_ctx):
         "category": {"type": "string", "description": "Optional category label (e.g. 'news', 'tech', 'reddit'). Omit if unsure."},
     },
     required=["url"],
+    requires="rss",
 )
 async def add_rss_feed_tool(*, user_id, arguments, **_ctx):
     import asyncio as _asyncio