Fix Dagre layout: split dbt snapshots into own group

[vbalalian]

Summary

• The omni_analytics group rendered side-by-side with dbt_models because snap_user_rfm (at Dagre rank 6 in dbt_models) shared the same rank as all omni_analytics assets, causing Dagre to lay the groups out horizontally instead of vertically.
• Uses conditional Jinja in the dbt translation config to assign snapshots to a dbt_snapshots group, keeping dbt_models at ranks 1–5 so omni_analytics flows below it.

Test plan

• CI passes (dg list defs, dbt build)
• Verify in Dagster UI that omni_analytics group renders below dbt_models instead of beside it
• Confirm snap_user_rfm appears in a dbt_snapshots group