Add risk-based position sizing (以损定仓) with per-open k resize.

Manual vs risk modes are exclusive; each open floors k to 1 decimal so estimated premium+fees stay within the loss budget.

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

backend/app/api/sim.py

@@ -96,9 +96,44 @@ async def sim_open_group(
     option_inst = (
         pick.pair.call_inst_id if option_side == "call" else pick.pair.put_inst_id
     )
+    # 强制方向时用该腿卖一估权利金；否则用选向结果
+    sizing_ask = float(
+        option_ask
+        if force in ("call", "put")
+        else pick.option_ask
+    )
 
     wkey = window_key()
     db = get_db()
+    from ..strategy.risk_sizing import apply_risk_sizing_to_ledger
+
+    rs = apply_risk_sizing_to_ledger(
+        index_px=float(pick.underlying_px),
+        option_ask=sizing_ask,
+        db=db,
+    )
+    if not rs.ok:
+        raise HTTPException(status_code=409, detail=rs.detail)
+
+    try:
+        from ..strategy.open_capacity import assess_open_capacity
+
+        cap = assess_open_capacity(db)
+        if cap.get("perp_can_open") is False or cap.get("option_can_open") is False:
+            detail = (
+                f"{cap.get('perp_label')} · {cap.get('option_label')}；"
+                f"永续需≈{cap.get('perp_need_usdt')}U/有{cap.get('perp_have_usdt')}U，"
+                f"期权需≈{cap.get('option_need_usdc')}U/有{cap.get('option_have_usdc')}U"
+            )
+            raise HTTPException(
+                status_code=409,
+                detail=f"资金不足，暂不可开新仓：{detail}",
+            )
+    except HTTPException:
+        raise
+    except Exception:
+        pass
+
     count = len(
         db.fetchall("SELECT group_id FROM groups WHERE group_id LIKE ?", (f"G-{wkey}-%",))
     )