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_pipeline.py

@@ -20,18 +20,26 @@ SLOT_NAMES = ("compilation", "morning", "midday", "afternoon")
 # ── External data gather ──────────────────────────────────────────────────────
 
 async def _gather_external(user_id: int) -> dict:
-    """Collect RSS items and weather."""
-    from fabledassistant.services.rss import get_recent_items
+    """Collect RSS items (when enabled) and weather."""
     from fabledassistant.services.weather import get_cached_weather
 
-    rss_items, weather = await asyncio.gather(
-        get_recent_items(user_id, limit=20),
-        get_cached_weather(user_id),
-        return_exceptions=True,
-    )
+    rss_on = (await get_setting(user_id, "rss_enabled", "false")).lower() == "true"
+    rss_items: list = []
+    if rss_on:
+        from fabledassistant.services.rss import get_recent_items
+        try:
+            rss_items = await get_recent_items(user_id, limit=20)
+        except Exception:
+            pass
+
+    try:
+        weather = await get_cached_weather(user_id)
+    except Exception:
+        weather = []
+
     return {
-        "rss_items": rss_items if not isinstance(rss_items, Exception) else [],
-        "weather": weather if not isinstance(weather, Exception) else [],
+        "rss_items": rss_items,
+        "weather": weather,
     }