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/shared/metadata.py

@@ -302,7 +302,11 @@ async def _persist_market_items(
     provider: str,
     content_hash: str,
 ) -> tuple[int, list[str]]:
-    """Persist market data items as market_snapshots rows."""
+    """Persist market data items as market_snapshots rows.
+
+    For grouped daily responses, each item contains a 'T' field with the
+    ticker. When present, the item's ticker overrides the job-level ticker.
+    """
     ids: list[str] = []
     for item in items:
         item_hash = content_hash_str(json.dumps(item, sort_keys=True))
@@ -313,11 +317,24 @@ async def _persist_market_items(
         if exists:
             continue
 
+        # Use item-level ticker if available (grouped daily), else job-level
+        item_ticker = item.get("T", ticker) or ticker
         snapshot_type = _infer_market_snapshot_type(item)
+
+        # For grouped daily items, look up company_id by ticker
+        item_company_id = company_id
+        if item.get("T") and not company_id:
+            cid = await pool.fetchval(
+                "SELECT id FROM companies WHERE ticker = $1", item_ticker
+            )
+            if cid:
+                item_company_id = str(cid)
+            # If not in our companies table, store with company_id=NULL
+
         row_id = await persist_market_snapshot(
             pool,
-            company_id=company_id,
-            ticker=ticker,
+            company_id=item_company_id,
+            ticker=item_ticker,
             snapshot_type=snapshot_type,
             data=item,
             source_provider=provider,