feat: add Polygon grouped daily endpoint for broad market data

Two tiers of market data:
1. Per-ticker prev bars (existing 50 sources, 15-min cadence) for
   watchlist detail — trading decisions, stop-loss, position sizing
2. Grouped daily (new single source, once per day) for broad market
   context — correlation analysis, sector rotation, competitive intel

Changes:
- Add grouped_daily endpoint to PolygonMarketAdapter with auto date
  calculation (previous trading day, skip weekends)
- Add fetch_global_market_sources() to scheduler for sources without
  company_id, scheduled once daily (86400s cadence)
- Update _persist_market_items to use item-level ticker from T field
  and look up company_id dynamically for grouped daily bars
- Migration 020: make company_id nullable on sources and
  market_snapshots tables, add grouped daily source row
- Fix backtest replay to query market_snapshots data->>'c' for prices

services/adapters/market_adapter.py

@@ -43,6 +43,7 @@ class PolygonMarketAdapter(MarketDataAdapter):
 
     PREV_BARS = "/v2/aggs/ticker/{ticker}/prev"
     RANGE_BARS = "/v2/aggs/ticker/{ticker}/range/{multiplier}/{timespan}/{from_date}/{to_date}"
+    GROUPED_DAILY = "/v2/aggs/grouped/locale/us/market/stocks/{date}"
     TICKER_DETAILS = "/v3/reference/tickers/{ticker}"
 
     def __init__(self, api_key: str, base_url: str = "https://api.polygon.io") -> None:
@@ -132,6 +133,20 @@ class PolygonMarketAdapter(MarketDataAdapter):
                 params["sort"] = config["sort"]
             if config.get("limit"):
                 params["limit"] = str(config["limit"])
+        elif endpoint_key == "grouped_daily":
+            # Grouped daily: returns bars for ALL tickers for a given date
+            target_date = config.get("date", "")
+            if not target_date:
+                # Default to previous trading day
+                from datetime import date, timedelta
+                today = date.today()
+                prev = today - timedelta(days=1)
+                # Skip weekends
+                while prev.weekday() > 4:
+                    prev -= timedelta(days=1)
+                target_date = prev.isoformat()
+            path = self.GROUPED_DAILY.format(date=target_date)
+            params["adjusted"] = str(config.get("adjusted", True)).lower()
         elif endpoint_key == "ticker_details":
             path = self.TICKER_DETAILS.format(ticker=ticker)
         else:
@@ -149,6 +164,7 @@ class PolygonMarketAdapter(MarketDataAdapter):
             return [results] if isinstance(results, dict) and results else []
 
         # Aggregate endpoints return results as a list
+        # For grouped_daily, each item has a "T" field with the ticker
         results = data.get("results", [])
         if isinstance(results, list):
             return results