Add 4-pair candle-boundary auto-scanner (free tier safe)

Expand from 2 to 4 pairs (EUR/USD, USD/JPY, GBP/USD, XAU/USD) with
automatic scanning at M15 and H1 candle close boundaries. Scheduler
staggers H1 by 60s to stay within 8 credits/min; daily budget is
480 of 800 allowed. Cache TTL is now candle-duration-aware so cross-TF
confirmation data is reused without extra API calls. Spread estimate
uses primary-TF data instead of fetching M5 separately.

data_provider.py

@@ -43,6 +43,12 @@ class ForexDataProvider:
         Timeframe.D1: 1, Timeframe.W1: 0.143,
     }
 
+    CANDLE_SECONDS = {
+        Timeframe.M1: 60, Timeframe.M5: 300, Timeframe.M15: 900,
+        Timeframe.M30: 1800, Timeframe.H1: 3600, Timeframe.H4: 14400,
+        Timeframe.D1: 86400, Timeframe.W1: 604800,
+    }
+
     RATE_LIMIT = 8
     RATE_WINDOW = 60.0
 
@@ -79,9 +85,10 @@ class ForexDataProvider:
         cache_key = f"{symbol}_{timeframe.value}"
         now = time.time()
 
+        cache_ttl = self.CANDLE_SECONDS.get(timeframe, self.config.refresh_interval)
         if (
             cache_key in self._cache_timestamps
-            and (now - self._cache_timestamps[cache_key]) < self.config.refresh_interval
+            and (now - self._cache_timestamps[cache_key]) < cache_ttl
             and cache_key in self._cache
         ):
             cached = self._cache.get(cache_key)