feat(iam): grant end-users access to activity queries (#189)

## Summary

We just released the new activity service, which brings support for
browsing activity timelines and querying events. Previously, the Viewer
role only had access to audit log queries. This updates it to include
the full activity viewer permissions so users can take advantage of the
new activity and event features too.

Editor and Owner roles inherit from Viewer, so they'll pick up these
permissions automatically.

## Test plan
- [ ] Assign a user the Viewer role and confirm they can query
activities, events, and audit logs
- [ ] Verify Editor and Owner users retain the same access

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Author: scotwells

config/assignable-organization-roles/roles/datum-cloud-viewer.yaml

@@ -24,7 +24,7 @@ spec:
       namespace: milo-system
     - name: iam-user-invitations-reader
       namespace: milo-system
-    - name: activity.miloapis.com-audit-log-querier
+    - name: activity.miloapis.com-viewer
       namespace: milo-system
     - name: notes-viewer
       namespace: milo-system