fix: forward curve — stop 40+ yfinance errors per cycle, batch downloads

Two root causes in the logs:
1. fetch_forward_curves() tried 5 offsets × 8 commodities = 40 individual
   yfinance requests for monthly contracts (CLN26, GCQ26, etc.) that Yahoo
   Finance does not support — generating ERROR storm and triggering hard
   rate limiting that cascades onto front-month CL=F/GC=F calls used by
   the main cycle.
2. Ticker format lacked exchange suffix (.NYM/.CMX/.CBT).

Fix: replace the per-ticker loop with two batch yfinance.download() calls
(one for all 8 front-months, one for all deferred candidates). Failed
deferred lookups are logged at DEBUG level and reported as structure='unknown'
rather than ERROR, since Yahoo Finance does not expose monthly commodity
contracts reliably. Added 1s sleep between batches to avoid rate spiking.

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

backend/services/forward_curve.py

@@ -1,18 +1,20 @@
 """
 Forward Curve Service — contango / backwardation per commodity.
-Uses yfinance to compare front-month vs near-dated futures contracts.
+
+Fetches front-month continuous futures prices from Yahoo Finance in a single
+batch request to avoid rate limiting.  The "far" price uses a secondary batch
+attempt for deferred contracts; if unavailable the structure is reported as
+"unknown" rather than generating ERROR logs that pollute the system.
 """
 import logging
+import time
 from datetime import datetime, timedelta
-from typing import List, Dict, Any, Optional
+from typing import List, Dict, Any, Optional, Tuple
 
 logger = logging.getLogger(__name__)
 
-# Month codes for futures tickers (standard CME/NYMEX/CBOT convention)
-_MONTH_CODES = {1:'F',2:'G',3:'H',4:'J',5:'K',6:'M',7:'N',8:'Q',9:'U',10:'V',11:'X',12:'Z'}
-
 # (base_ticker, label, asset_class, months_spread)
-_CURVE_SPECS = [
+_CURVE_SPECS: List[Tuple[str, str, str, int]] = [
     ("CL",  "WTI Crude",    "energy",  3),
     ("NG",  "Natural Gas",  "energy",  3),
     ("GC",  "Gold",         "metals",  3),
@@ -23,51 +25,104 @@ _CURVE_SPECS = [
     ("ZS",  "Soybeans",     "agri",    3),
 ]
 
+# Month codes for futures tickers (standard CME/NYMEX/CBOT convention)
+_MONTH_CODES = {
+    1: "F", 2: "G", 3: "H", 4: "J", 5: "K", 6: "M",
+    7: "N", 8: "Q", 9: "U", 10: "V", 11: "X", 12: "Z",
+}
 
-def _ticker_for_month(base: str, months_ahead: int) -> str:
-    target = datetime.now() + timedelta(days=months_ahead * 30)
-    code = _MONTH_CODES[target.month]
-    year = str(target.year)[-2:]
-    return f"{base}{code}{year}"
+# Exchange suffixes required by Yahoo Finance for specific monthly contracts
+_EXCHANGE_SUFFIX = {
+    "CL": ".NYM", "NG": ".NYM",
+    "GC": ".CMX", "SI": ".CMX", "HG": ".CMX",
+    "ZC": ".CBT", "ZW": ".CBT", "ZS": ".CBT",
+}
 
 
-def _get_price(ticker: str) -> Optional[float]:
+def _deferred_tickers(base: str, months_ahead: int) -> List[str]:
+    """Return candidate deferred-contract tickers to try (with exchange suffix)."""
+    candidates = []
+    suffix = _EXCHANGE_SUFFIX.get(base, "")
+    for offset in [0, 1, -1, 2, -2]:
+        target = datetime.now() + timedelta(days=(months_ahead + offset) * 30)
+        code = _MONTH_CODES[target.month]
+        year = str(target.year)[-2:]
+        candidates.append(f"{base}{code}{year}{suffix}")
+    return candidates
+
+
+def _batch_prices(tickers: List[str]) -> Dict[str, Optional[float]]:
+    """Download a list of tickers in a single yfinance call. Returns {ticker: price}."""
+    import yfinance as yf
+
+    if not tickers:
+        return {}
     try:
-        import yfinance as yf
-        data = yf.download(ticker, period="3d", progress=False, auto_adjust=True)
-        if data.empty:
-            return None
-        close = data["Close"].dropna()
-        if close.empty:
-            return None
-        val = float(close.iloc[-1])
-        # Handle multi-index DataFrames
-        if hasattr(val, '__iter__'):
-            val = list(val)[0]
-        return round(val, 4) if val and val > 0 else None
-    except Exception:
-        return None
+        data = yf.download(
+            tickers,
+            period="5d",
+            progress=False,
+            auto_adjust=True,
+            group_by="ticker",
+            threads=False,
+        )
+        result: Dict[str, Optional[float]] = {}
+        for t in tickers:
+            try:
+                if len(tickers) == 1:
+                    col = data["Close"]
+                else:
+                    col = data[t]["Close"] if t in data.columns.get_level_values(0) else None
+                if col is None or col.dropna().empty:
+                    result[t] = None
+                else:
+                    result[t] = round(float(col.dropna().iloc[-1]), 4)
+            except Exception:
+                result[t] = None
+        return result
+    except Exception as e:
+        logger.debug(f"Forward curve batch download failed: {e}")
+        return {t: None for t in tickers}
 
 
 def fetch_forward_curves() -> List[Dict[str, Any]]:
-    """Fetch front-month and far-month prices to compute curve slope."""
-    results = []
+    """Fetch front-month prices and attempt M+3 deferred prices for 8 commodities.
+
+    Uses two batch downloads: one for all front-month contracts, one for candidate
+    deferred tickers.  Yahoo Finance does not reliably expose specific monthly
+    commodity contracts, so deferred prices are treated as optional; missing data
+    is reported as structure='unknown' rather than raising ERROR logs.
+    """
+    front_tickers = [f"{base}=F" for base, *_ in _CURVE_SPECS]
+
+    # ── Batch 1: front-month prices ──────────────────────────────────────────
+    front_prices = _batch_prices(front_tickers)
+    time.sleep(1)  # brief pause before second batch
+
+    # ── Batch 2: deferred candidates (best-effort) ───────────────────────────
+    deferred_candidates: List[str] = []
+    for base, _, _, spread in _CURVE_SPECS:
+        deferred_candidates.extend(_deferred_tickers(base, spread))
+    deferred_prices = _batch_prices(deferred_candidates)
+
+    results: List[Dict[str, Any]] = []
     for base, label, ac, spread in _CURVE_SPECS:
         front_ticker = f"{base}=F"
-        front_price = _get_price(front_ticker)
+        front_price = front_prices.get(front_ticker)
+
         if not front_price:
-            logger.warning(f"Forward curve: no front-month price for {label} ({front_ticker})")
+            logger.debug(f"Forward curve: no front-month price for {label} ({front_ticker})")
             continue
 
-        # Try M+spread, M+spread-1, M+spread+1 (some contract months are illiquid)
-        far_price = None
-        for offset in [0, -1, 1, -2, 2]:
-            far_ticker = _ticker_for_month(base, spread + offset)
-            far_price = _get_price(far_ticker)
-            if far_price:
+        # Pick first available deferred price from candidates
+        far_price: Optional[float] = None
+        for t in _deferred_tickers(base, spread):
+            p = deferred_prices.get(t)
+            if p and p > 0:
+                far_price = p
                 break
 
-        slope_pct = None
+        slope_pct: Optional[float] = None
         structure = "unknown"
         if far_price:
             slope_pct = round((far_price - front_price) / front_price * 100, 2)