Restore MEMP/MPAT in Bank enum for DB compatibility

Existing account rows reference these values. Enum must stay compatible
with data in production. Only the frontend and seed data were cleaned up.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/app/models/models.py

@@ -46,6 +46,8 @@ class Bank(str, enum.Enum):
     FCL = "FCL"
     ROP = "ROP"
     VOL = "VOL"
+    MEMP = "MEMP"
+    MPAT = "MPAT"
     MORTGAGE = "MORTGAGE"