feat: ticker validation + SLV/USO/WEAT/CORN/TUR added to ETFs watchlist

Backend:
- GET /api/market/validate?symbol= — validates ticker against yfinance,
  returns {valid, name, price} or {valid: false, reason: 'helpful message'}
- Added SLV, USO, WEAT, CORN, TUR to ETFs WATCHLIST category

Frontend:
- validateTicker() async helper exported from useApi.ts
- InstrumentPicker (PatternLab): custom ticker field now validates before selecting
  Shows spinner while checking, red error message if not found on yfinance
- InstrumentLens (PatternExplorer): same validation on Go button + Enter key

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

backend/routers/market_data.py

@@ -39,6 +39,24 @@ def watchlist():
     return WATCHLIST
 
 
+@router.get("/validate")
+def validate_ticker(symbol: str = Query(..., description="Ticker to validate against yfinance")):
+    """Check if a ticker is fetchable. Returns valid=True + live price/name, or valid=False + reason."""
+    from services.data_fetcher import get_quote
+    import yfinance as yf
+    symbol = symbol.strip().upper()
+    q = get_quote(symbol)
+    if q and q.get("price"):
+        # Try to get a display name
+        try:
+            info = yf.Ticker(symbol).fast_info
+            name = getattr(info, "long_name", None) or getattr(info, "short_name", None) or symbol
+        except Exception:
+            name = symbol
+        return {"valid": True, "symbol": symbol, "name": name, "price": q["price"], "change_pct": q.get("change_pct")}
+    return {"valid": False, "symbol": symbol, "reason": f"Ticker '{symbol}' not found on yfinance — check the symbol (e.g. GC=F for Gold, ^GSPC for S&P 500)"}
+
+
 @router.get("/macro-regime")
 def macro_regime(force: bool = False):
     """Macro gauge values + 5-scenario scoring. Cached 15 min."""