Add dagster-omni integration for Omni workbook assets

[vbalalian]

Summary

• Adds dagster-omni==0.28.18 and scaffolds an OmniComponent to represent Omni Analytics workbooks as Dagster assets
• Adds Omni component state refresh (dg utils refresh-component-state) to the CD pipeline deploy step
• Adds OMNI_API_KEY and OMNI_BASE_URL to Terraform variables, startup script systemd overrides, and tfvars example
• Adds YAML validation test for the Omni component definition
• Documents the integration in README, orchestration docs, and dbt exposures

Test plan

• CI passes (dagster lint, test, validate; dbt; terraform)
• After merge, SSH into VM to add OMNI_API_KEY and OMNI_BASE_URL to existing systemd overrides
• Run initial dg utils refresh-component-state on VM to populate component state
• Verify Omni assets appear in Dagster UI lineage graph
• Trigger CD and confirm state refresh step succeeds

[github-actions]

Omni Content Validator Results

Content Validator did not produce a report.