Add system alerts log with nav badge and Admin Alerts tab.

Persist job and ingestion warnings/errors for 7 days, surface a dismissible top-nav badge, treat stale OHLCV as a warning (e.g. ticker renames), and show market bar age on the ticker freshness chip.

app/models/__init__.py

@@ -13,6 +13,7 @@ from app.models.paper_trade import PaperTrade
 from app.models.regime_snapshot import RegimeSnapshot
 from app.models.benchmark_price import BenchmarkPrice
 from app.models.signal_context_snapshot import SignalContextSnapshot
+from app.models.system_event import SystemEvent
 
 __all__ = [
     "Ticker",
@@ -32,4 +33,5 @@ __all__ = [
     "RegimeSnapshot",
     "BenchmarkPrice",
     "SignalContextSnapshot",
+    "SystemEvent",
 ]