`identities_v2` table has a lot of overrides for deleted features

After https://github.com/Flagsmith/flagsmith/pull/5080, we did not retroactively update the table, which resulted in a large number of stale override data which affects limits/pagination.

We need a management command that iterates over every v2 document, cross-checks the key with current feature ids in the main environment document, and removes the document if the key is stale.

Start by locating the management-command entry point and the code representing the identities_v2 and main environment documents. Verify how feature IDs are compared, then confirm that stale v2 documents are removed and no longer affect limits or pagination.

Written by the indexing model from the issue text.