feat: SuperAdmin bootstrap, i18n sync fix and AssetAssignment ORM fix

- Fixed AttributeError in User model (added region_code, preferred_language)
- Fixed InvalidRequestError in AssetAssignment (added organization relationship)
- Configured STATIC_DIR for translation sync
- Applied Alembic migrations for user schema updates

backend/static/locales/en.json

@@ -0,0 +1,23 @@
+{
+  "AUTH": {
+    "LOGIN": {
+      "SUCCESS": "Login successful. Welcome back!"
+    },
+    "ERROR": {
+      "EMAIL_EXISTS": "This email is already registered."
+    }
+  },
+  "SENTINEL": {
+    "LOCK": {
+      "MSG": "Account locked for security reasons."
+    }
+  },
+  "EMAIL": {
+    "REG": {
+      "SUBJECT": "Confirm your registration"
+    },
+    "PWD_RESET": {
+      "SUBJECT": "Password Reset Request"
+    }
+  }
+}