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).

pair_signals/__init__.py

@@ -30,6 +30,7 @@ def produce_signal(
     primary_tf: Timeframe,
     data_provider: BaseDataProvider,
     confirmation_tfs: Optional[List[Timeframe]] = None,
+    log_source: str = "dashboard",
 ) -> SignalReport:
     config_factory = PAIR_PRODUCERS.get(symbol)
     if config_factory is None:
@@ -41,7 +42,7 @@ def produce_signal(
     config = config_factory(primary_tf, confirmation_tfs)
     aggregator = SignalAggregator(config)
     aggregator.data_provider = data_provider
-    return aggregator.analyze_pair(symbol=symbol)
+    return aggregator.analyze_pair(symbol=symbol, log_source=log_source)
 
 
 from pair_signals import xauusd as _xau_mod