signal_log: persist every live scan (signal, grade, price, news) to SQLite

New signal_log.py logs each SignalReport to logs/signals.db via a
fail-safe hook in SignalAggregator.analyze_pair — the live-only entry
point used by the bot, daemon, CLI and dashboard. Backtester calls
analyze_from_dataframes directly, so live==backtest parity is preserved
(test-locked). Rows carry signal/grade/price/spread, MTF + trade-setup +
reasons/warnings JSON, and news-at-scan-time (calendar events + scored
headlines JSON via NewsEngine class-level caches).

Config: signal_log block (enabled, path; env SIGNAL_LOG_ENABLED/PATH),
source tagged per consumer (telegram/oanda_daemon/cli/dashboard).
381 tests pass (24 modules).

oanda_daemon.py

@@ -466,7 +466,8 @@ class TradingDaemon:
 
         aggregator = SignalAggregator(config)
         aggregator.data_provider = self._provider
-        report = aggregator.analyze_pair(symbol=SYMBOL)
+        report = aggregator.analyze_pair(symbol=SYMBOL,
+                                         log_source="oanda_daemon")
 
         logger.info(
             "%s scan: signal=%s score=%.1f conf=%.0f quality=%s "