Extends ReviewRequestIndex and UserIndex with new Elasticsearch-specific

fields required by the faceted search engine. These fields are ignored by

Whoosh.
ReviewRequestIndex gains:

- status, branch — CharField(faceted=True) for status and branch

  filters and aggregations

- repository_id, submitter_id — IntegerField FKs for the repository

  and author searchable filters

- date_created, date_updated — datetime fields for creation and

  last-updated range filters

- target_reviewer_ids, reviewer_user_ids, commenter_user_ids —

  multi-value integer fields for reviewer and commenter filters

- has_ship_it, has_reviews, has_issues_open, has_file_attachment,

has_depends_on — BooleanField for option-count term queries

- comment_issue_statuses, file_attachment_types — multi-value fields

  for issue status and file type filters
The review-based boolean fields (has_ship_it, has_reviews,

reviewer_user_ids) use the prefetched _public_reviews attribute when

available during rebuild_index and fall back to direct DB queries during

on-the-fly indexing. has_issues_open and commenter_user_ids always use

direct DB queries since comment volumes can be very large.
UserIndex gains:

- is_active, is_staff — BooleanField(faceted=True) for user status

  and role filters

- group_ids — multi-value field for group membership filter

- date_joined, last_login — datetime fields for date range filters

- username_display — stored keyword copy of username for terms

  aggregations
UserIndex.index_queryset now indexes all users regardless of their

is_active status (the is_active=True filter has been removed).

Inactive users now appear in search results by default and can be filtered

out using the is_active faceted field.
A new GroupIndex is introduced to support the groups search tab. It

indexes:

- has_open_review_requests, in_active_reviews — BooleanField for

  filtering groups by review request activity

- member_count — IntegerField for filtering groups by membership size

- invite_only — BooleanField used by the permission filter to exclude

  groups the searching user cannot access
A rebuild_index is required after upgrading to populate these new fields.

Ran full test suite.

Verified that each new prepare_* method on ReviewRequestIndex and

UserIndex returns the correct value, including fallback behaviour for

on-the-fly indexing when prefetches are not available. Confirmed that

index_queryset on both indexes does not introduce N+1 queries for any of the

new fields.