fix: set gate_passed=True for ALT track in save_feature_event (NOT NULL constraint)

backend/signal_engine.py

@@ -1038,7 +1038,7 @@ def save_feature_event(ts: int, symbol: str, result: dict, strategy: str):
     score_crowding  = (f.get("crowding")  or {}).get("score", 0) if track == "ALT" else None
     score_env       = (f.get("environment") or {}).get("score", 0) if track == "ALT" else None
     score_aux       = (f.get("auxiliary") or {}).get("score", 0) if track == "ALT" else None
-    gate_passed     = f.get("gate_passed") if track == "BTC" else None
+    gate_passed     = f.get("gate_passed") if track == "BTC" else True  # ALT轨无gate概念，视为通过
     block_reason    = f.get("block_reason") if track == "BTC" else None
 
     with get_sync_conn() as conn: