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).

main.py

@@ -336,7 +336,7 @@ def build_config(args) -> PlatformConfig:
 def run_live(args, config: PlatformConfig):
     print(f"{Colors.GRAY}  Fetching market data and running analysis...{Colors.RESET}\n")
     aggregator = SignalAggregator(config)
-    report = aggregator.analyze_pair(symbol=config.symbol)
+    report = aggregator.analyze_pair(symbol=config.symbol, log_source="cli")
 
     # Explicit user intent: force bypasses the enabled/surface/quality
     # toggles, but the daily panel budget and LLM cap still apply.