Audit: enrich EnvironmentFeatureVersion-published log with changed feature states

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.