fix: reduce Alpaca multi-bar batch_size 200 → 100

Community reports 502 above ~100 symbols per request. Official docs have
no stated limit but 200 appears to be a ceiling. 100 is a safe conservative
default that avoids the 502 edge case with minimal performance impact
(500 tickers = 5 batches instead of 3).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

app/services/alpaca_client.py

@@ -215,7 +215,7 @@ class AlpacaClient:
         start: Optional[str] = None,
         end: Optional[str] = None,
         limit: int = 10000,
-        batch_size: int = 200,
+        batch_size: int = 100,
         feed: Optional[str] = None,
     ) -> Dict[str, List[Dict]]:
         """
@@ -231,7 +231,7 @@ class AlpacaClient:
             start: RFC-3339 date/datetime string
             end: RFC-3339 date/datetime string
             limit: Max bars per page (Alpaca max 10000)
-            batch_size: Max symbols per Alpaca request (default 200, ~1KB URL)
+            batch_size: Max symbols per Alpaca request (default 100, conservative safe limit)
 
         Returns:
             Dict mapping normalized Alpaca symbol → list of bar dicts