Add database indexes for `authentik_core_user.email`

Add database indexes for `authentik_core_user.email`, including case-insensitive lookups such as `UPPER(email::text)`.

Adding appropriate indexes would improve lookup performance and reduce unnecessary database load for deployments that use email as a primary user identifier.

Start by locating the model and migration that define authentik_core_user, then inspect the existing email lookup patterns, including the UPPER(email::text) expression named in the issue. Done means appropriate indexes support case-insensitive email lookups and the resulting queries reduce unnecessary database load.

Written by the indexing model from the issue text.