refactor: separate Sentiment and Technical desks — no signal overlap

- Mark 6 price signals (ma_cross, rsi_extreme, bb_squeeze, new_52w_extreme, price_gap, volume_spike) as desk_type=technical so they no longer appear in the Sentiment desk
- Sentiment desk now shows MacroGaugeSelector (32 gauges grouped by bloc: Liquidité, Crédit, Volatilité, Métaux…) instead of the stock/ETF instrument picker
- Sentiment desk instruments seed updated to macro gauge keys (vix, vvix, skew, hyg, dxy, slope_10y3m, gold_copper_ratio)
- Signal init useEffect extended to cover sentiment desk as well as technical

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/services/database.py

@@ -1098,7 +1098,7 @@ def init_db():
                 "SKEW > 140 = marché paye cher pour la protection downside. "
                 "Inversion terme structure VIX (front > back) = peur concentrée court terme."
             ),
-            "instruments": json.dumps(["SPY","QQQ","VXX","TLT","HYG","GLD"]),
+            "instruments": json.dumps(["vix","vvix","skew","hyg","dxy","slope_10y3m","gold_copper_ratio"]),
             "config": json.dumps({
                 "lookback_days": 5,
                 "signals": {