Handle Finnhub 403; make exchange prefix configurable

OANDA forex candles require a paid plan — free-tier keys get 403.

- Add FINNHUB_FOREX_EXCHANGE env var (default OANDA) so the exchange
  prefix can be changed without touching code
- Handle 403 specifically: clear error message tells the user which
  exchange failed and how to discover what their key can access
- Add discover_exchanges() method wrapping forex_exchanges() API call
  so the right prefix can be found before configuring the bot

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

data_provider_finnhub.py

@@ -57,6 +57,10 @@ class FinnhubDataProvider:
     H4 bars are synthesised client-side by resampling 60-minute candles.
     """
 
+    # Forex exchange/broker prefix — override with FINNHUB_FOREX_EXCHANGE env var.
+    # OANDA requires a paid plan. Run discover_exchanges() to see what your key can access.
+    DEFAULT_EXCHANGE = "OANDA"
+
     # Finnhub resolution strings (H4 uses 60-min candles, then resampled)
     FH_RESOLUTIONS = {
         Timeframe.M1:  "1",
@@ -80,14 +84,41 @@ class FinnhubDataProvider:
                 "No Finnhub API key — set the FINNHUB_API_KEY environment variable."
             )
         self._client = finnhub.Client(api_key=api_key)
+        self._exchange = (
+            os.environ.get("FINNHUB_FOREX_EXCHANGE") or self.DEFAULT_EXCHANGE
+        )
+        logger.info(f"Finnhub forex exchange: {self._exchange}")
         self._cache: Dict[str, pd.DataFrame] = {}
         self._cache_timestamps: Dict[str, float] = {}
 
-    @staticmethod
-    def _to_finnhub_symbol(symbol: str) -> str:
-        """Convert EURUSD=X → OANDA:EUR_USD."""
+    def _to_finnhub_symbol(self, symbol: str) -> str:
+        """Convert EURUSD=X → {exchange}:EUR_USD."""
         clean = symbol.replace("=X", "")
-        return f"OANDA:{clean[:3]}_{clean[3:]}"
+        return f"{self._exchange}:{clean[:3]}_{clean[3:]}"
+
+    def discover_exchanges(self) -> List[str]:
+        """
+        Return the list of forex exchanges available on your Finnhub plan.
+        Call this once to find which exchange prefix to use.
+
+        Usage:
+            set FINNHUB_API_KEY=your_key
+            python -c "
+            from config import PlatformConfig
+            from data_provider_finnhub import FinnhubDataProvider
+            p = FinnhubDataProvider(PlatformConfig())
+            print(p.discover_exchanges())
+            "
+        """
+        self._throttle()
+        try:
+            exchanges = self._client.forex_exchanges()
+            logger.info(f"Available Finnhub forex exchanges: {exchanges}")
+            return exchanges
+        except finnhub.FinnhubAPIException as e:
+            raise DataUnavailableError(
+                f"Could not list Finnhub forex exchanges: {e}"
+            ) from e
 
     # ── Public interface (matches ForexDataProvider) ───────────────────
 
@@ -256,6 +287,12 @@ class FinnhubDataProvider:
                     "Finnhub API rate limit reached — too many requests. "
                     "Free tier allows 60 calls/minute. Try again shortly."
                 ) from e
+            if e.status_code == 403:
+                raise DataUnavailableError(
+                    f"Finnhub 403: exchange '{self._exchange}' is not accessible on your plan. "
+                    "Run discover_exchanges() to see what your API key can access, "
+                    "then set FINNHUB_FOREX_EXCHANGE=<exchange> and restart."
+                ) from e
             raise DataUnavailableError(
                 f"Finnhub API error for {fh_symbol} ({e.status_code}): {e}"
             ) from e