Show expiry settlement as index vs strike intrinsic.

Persist settle_index_px and surface formula in trade detail so expiry closes are not mistaken for book fills.

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

backend/app/models/db.py

@@ -42,7 +42,8 @@ CREATE TABLE IF NOT EXISTS groups (
   slip_cost REAL DEFAULT 0,
   note TEXT,
   exec_mode TEXT,
-  funding_usdt REAL
+  funding_usdt REAL,
+  settle_index_px REAL
 );
 
 CREATE TABLE IF NOT EXISTS fills (
@@ -152,6 +153,7 @@ class Database:
             for table, col, decl in (
                 ("groups", "exec_mode", "TEXT"),
                 ("groups", "funding_usdt", "REAL"),
+                ("groups", "settle_index_px", "REAL"),
                 ("fills", "exec_mode", "TEXT"),
                 ("fills", "fee_ccy", "TEXT"),
             ):