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/briefing_scheduler.py

@@ -362,10 +362,12 @@ async def _run_slot_for_user(user_id: int, slot: str) -> None:
         # Refresh external data first
         try:
             import json
-            from fabledassistant.services.rss import refresh_all_feeds
             config_raw = await get_setting(user_id, "briefing_config", "{}")
             config = json.loads(config_raw) if isinstance(config_raw, str) else {}
-            await refresh_all_feeds(user_id)
+            rss_on = (await get_setting(user_id, "rss_enabled", "false")).lower() == "true"
+            if rss_on:
+                from fabledassistant.services.rss import refresh_all_feeds
+                await refresh_all_feeds(user_id)
             from fabledassistant.services import weather as wx
             for key, loc in config.get("locations", {}).items():
                 if loc.get("lat") and loc.get("lon"):