oanda: retry once on transient 401/5xx/network faults in candle fetch

The practice API rejects ~15-20 requests/day with a spurious 401
"Insufficient authorization" while the same token succeeds on adjacent
requests; each one silently cost a full scan cycle on that timeframe.
One retry after RETRY_DELAY_S (2 s) absorbs these plus transient
5xx/network faults. Real credential failures fail both attempts and
raise as before; other 4xx (bad request, 429) are never retried.

Tests: transient-401 recovery, persistent-401 raise, 400 not retried.

Claude-Session: https://claude.ai/code/session_01Tujtmmhxj9mxKjPFSoVjmP

providers/oanda.py

@@ -8,6 +8,7 @@ Oanda's candle JSON, and exposes the real bid/ask spread from MBA candles.
 """
 
 import logging
+import time
 from datetime import datetime
 from typing import Dict, Optional
 
@@ -31,6 +32,7 @@ class OandaDataProvider(BaseDataProvider):
 
     name = "oanda"
     DATA_ERROR = OandaDataError
+    RETRY_DELAY_S = 2.0
 
     OANDA_GRANULARITY = {
         Timeframe.M1: "M1",
@@ -124,11 +126,35 @@ class OandaDataProvider(BaseDataProvider):
         else:
             params["count"] = min(count, self.MAX_BARS_PER_REQUEST)
 
-        try:
-            resp = self._session.get(url, params=params, timeout=30)
-        except requests.RequestException as e:
-            raise OandaDataError(
-                f"Oanda candle fetch failed for {instrument}: {e}") from e
+        # One retry absorbs Oanda's sporadic spurious 401s (~15-20/day on
+        # the practice API — the same token succeeds on adjacent requests)
+        # plus transient 5xx/network faults. A real credential problem
+        # fails both attempts and raises exactly as before. Other 4xx
+        # (bad request, 429) are never retried.
+        last_err: Optional[OandaDataError] = None
+        for attempt in range(2):
+            if attempt:
+                time.sleep(self.RETRY_DELAY_S)
+            try:
+                resp = self._session.get(url, params=params, timeout=30)
+            except requests.RequestException as e:
+                last_err = OandaDataError(
+                    f"Oanda candle fetch failed for {instrument}: {e}")
+                last_err.__cause__ = e
+                continue
+            if resp.status_code == 401 or resp.status_code >= 500:
+                last_err = OandaDataError(
+                    f"Oanda candle fetch failed for {instrument}: "
+                    f"HTTP {resp.status_code}: {resp.text[:300]}")
+                if attempt == 0:
+                    logger.warning(
+                        f"Oanda fetch for {instrument} got HTTP "
+                        f"{resp.status_code}; retrying in "
+                        f"{self.RETRY_DELAY_S}s")
+                continue
+            break
+        else:
+            raise last_err
 
         if resp.status_code != 200:
             raise OandaDataError(