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.

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

signal_aggregator.py

@@ -297,14 +297,12 @@ class SignalAggregator:
             report.news_sentiment = 0.0
             report.news_description = ""
 
-        # ── 5. Spread estimate (only meaningful in live mode) ──────
-        if use_news:
-            try:
-                report.spread_estimate = self.data_provider.get_spread_estimate(symbol)
-            except Exception:
-                report.spread_estimate = 0.0
-        else:
-            # Backtest sets spread externally via BacktestConfig
+        # ── 5. Spread estimate from primary data (avoids extra API call) ─
+        try:
+            report.spread_estimate = float(
+                (primary_df["High"] - primary_df["Low"]).median()
+            )
+        except Exception:
             report.spread_estimate = 0.0
 
         # ── 6. Final signal ────────────────────────────────────────