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/routes/briefing.py

@@ -54,11 +54,17 @@ async def put_config():
     return jsonify({"ok": True})
 
 
+async def _rss_enabled() -> bool:
+    return (await get_setting(g.user.id, "rss_enabled", "false")).lower() == "true"
+
+
 # ── RSS Feeds ─────────────────────────────────────────────────────────────────
 
 @briefing_bp.route("/feeds", methods=["GET"])
 @_REQUIRE
 async def list_feeds():
+    if not await _rss_enabled():
+        return jsonify([])
     async with async_session() as session:
         result = await session.execute(
             select(RssFeed).where(RssFeed.user_id == g.user.id).order_by(RssFeed.id)
@@ -70,6 +76,8 @@ async def list_feeds():
 @briefing_bp.route("/feeds", methods=["POST"])
 @_REQUIRE
 async def add_feed():
+    if not await _rss_enabled():
+        return jsonify({"error": "RSS is disabled"}), 403
     data = await request.get_json()
     url = (data.get("url") or "").strip()
     if not url:
@@ -120,6 +128,8 @@ async def delete_feed(feed_id: int):
 @briefing_bp.route("/feeds/refresh", methods=["POST"])
 @_REQUIRE
 async def refresh_feeds():
+    if not await _rss_enabled():
+        return jsonify({"feeds_refreshed": 0, "new_items": 0})
     results = await rss_svc.refresh_all_feeds(g.user.id)
     total_new = sum(results.values())
     return jsonify({"feeds_refreshed": len(results), "new_items": total_new})
@@ -128,6 +138,8 @@ async def refresh_feeds():
 @briefing_bp.route("/feeds/recent", methods=["GET"])
 @_REQUIRE
 async def recent_items():
+    if not await _rss_enabled():
+        return jsonify({"items": []})
     limit = min(int(request.args.get("limit", 20)), 100)
     items = await rss_svc.get_recent_items(g.user.id, limit=limit)
     return jsonify({"items": items})
@@ -454,6 +466,9 @@ async def list_news():
       offset  — pagination offset (default 0)
       feed_id — optional integer filter by feed
     """
+    if not await _rss_enabled():
+        return jsonify({"items": [], "total": 0})
+
     from sqlalchemy import text as _text
 
     days = min(int(request.args.get("days", 2)), 90)