Add one-expiry-per-day open rhythm gate (default on).

Co-authored-by: Cursor <cursoragent@cursor.com>

backend/app/strategy/session.py

@@ -295,7 +295,29 @@ class StrategySession:
             logger.info("no expiry with hours>=%.1f", min_hours)
             return None
 
+        used_expiries: set[str] = set()
+        one_exp_day = bool(s.one_expiry_per_day)
+        try:
+            from ..models.db import get_db
+            from .clock import used_expiry_ymds_for_day
+
+            db = get_db()
+            one_exp_day = _as_bool_setting(
+                db.get_setting("one_expiry_per_day", str(s.one_expiry_per_day)),
+                s.one_expiry_per_day,
+            )
+            if one_exp_day:
+                used_expiries = used_expiry_ymds_for_day(db)
+        except Exception:
+            logger.exception("one_expiry_per_day lookup failed; continue without skip")
+
         for ymd in eligible:
+            if one_exp_day and ymd in used_expiries:
+                logger.info(
+                    "skip expiry=%s: already used today (one_expiry_per_day)",
+                    ymd,
+                )
+                continue
             pair = select_option_pair(
                 contracts,
                 mark_px=underlying,