fix(user): filter out soft-deleted memberships and organizations

Add get_active_memberships() method to User model that filters out
soft-deleted memberships and memberships of deleted organizations.
Update all usages of organization_memberships to use this method,
ensuring consistent handling of soft-deleted records across the
codebase. Also add deleted_at filters to CA queries in SSH helpers.

gatehouse_app/models/user/user.py

@@ -116,9 +116,24 @@ class User(BaseModel):
             is not None
         )
 
+    def get_active_memberships(self):
+        """Get active (non-deleted) organization memberships with active organizations.
+
+        Returns:
+            List of OrganizationMember instances where:
+            - membership.deleted_at is None
+            - organization exists and organization.deleted_at is None
+        """
+        return [
+            m for m in self.organization_memberships
+            if m.deleted_at is None
+            and m.organization
+            and m.organization.deleted_at is None
+        ]
+
     def get_organizations(self):
-        """Get all organizations the user is a member of."""
-        return [membership.organization for membership in self.organization_memberships]
+        """Get all active organizations the user is a member of."""
+        return [membership.organization for membership in self.get_active_memberships()]
 
     def has_totp_enabled(self) -> bool:
         """Check if user has TOTP enabled and verified.