Audit: enrich EnvironmentFeatureVersion-published log with changed feature states

[khvn26]

In v2 versioning environments, every flag change is published via an EnvironmentFeatureVersion, and the resulting audit row uses a fixed template (api/audit/constants.py:64):

ENVIRONMENT_FEATURE_VERSION_PUBLISHED_MESSAGE = "New version published for feature: %s"

That string is the entire log field. Nothing indicates which feature states changed, the enabled/disabled flip, segment overrides touched, or value diffs.

Five integrations consume this row via the AuditLog post_save filter (api/audit/signals.py:118-125, accepting FEATURE, FEATURE_STATE, SEGMENT, EF_VERSION) and render the log text:

• Datadog, New Relic, Slack, Grafana — render audit_log.log directly
• Dynatrace — same, plus audited_instance.feature.name for deployment name

v1 produces per-FS rows like Flag state updated: my_flag: Enabled. v2 collapses these into one row per version with no per-FS content.

Proposal:

• Render the EFV message via get_updated_feature_states_for_version(efv), enumerating each FS's action.
• Optionally expose the change list as structured audit-log metadata so future receivers don't re-parse prose.