Two targeted changes to existing files:

- ElasticsearchBackend.get_es_client() — adds a new method to the

  Elasticsearch search backend that constructs and returns a raw

elasticsearch.Elasticsearch client using the URL from the current backend

  configuration. Used by FacetedSearchEngine to run direct ES queries outside

  of Haystack. Raises RuntimeError if the installed elasticsearch package

  is unsupported.

- SignalProcessor — extends handle_save and handle_delete to call

FacetCache().invalidate_group(group_id) after each successful index update.

  A _MODEL_TO_GROUP mapping at module level translates the model class name

  to its facet cache group ID, covering ReviewRequest, User, Profile,

Group, and all comment model types (Comment, GeneralComment,

FileAttachmentComment, ScreenshotComment). Unrecognised model types are

  silently ignored. Cache invalidation is skipped when the index update itself

  raises an exception.
Group.users M2M changes are also handled: when users are added to or removed

from a group the affected user records are re-indexed and the user group's

cache is invalidated.

Verified that get_es_client returns a correctly configured Elasticsearch

client and raises RuntimeError for unsupported package versions. Verified

that SignalProcessor invalidates the correct facet cache group after save and

delete for all tracked model types, silently ignores unrecognised models, and

does not propagate indexing exceptions. Confirmed existing backend and signal

processor behaviour is unchanged.