Fix LIVE open/close double-book and security audit findings.

Prevent expiry dual-close from re-booking option cash, abandon ledger rejection, margin-mode mismatch, and manual close races; harden fill wait and refuse default AUTH_SECRET on LIVE.

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

backend/app/models/db.py

@@ -43,7 +43,8 @@ CREATE TABLE IF NOT EXISTS groups (
   note TEXT,
   exec_mode TEXT,
   funding_usdt REAL,
-  settle_index_px REAL
+  settle_index_px REAL,
+  perp_margin_mode TEXT
 );
 
 CREATE TABLE IF NOT EXISTS fills (
@@ -164,6 +165,7 @@ class Database:
                 ("groups", "exec_mode", "TEXT"),
                 ("groups", "funding_usdt", "REAL"),
                 ("groups", "settle_index_px", "REAL"),
+                ("groups", "perp_margin_mode", "TEXT"),
                 ("fills", "exec_mode", "TEXT"),
                 ("fills", "fee_ccy", "TEXT"),
             ):