Switch CMP configuration from Datamailer to Relay

[alexeygrigorev]

Summary

• replace the active CMP integration settings with RELAY_*
• remove all fallback to legacy DATAMAILER_* settings
• point documentation and examples at https://relay.dtcdev.click
• keep historical database model/table names so existing outbox and audit data is preserved
• add a regression test proving legacy settings no longer enable the integration

Companion infrastructure change: DataTalksClub/aws-infra#20

Verification

• uv run ruff check .
• uv run python manage.py test (758 tests)

Merge gate

Do not merge or deploy this PR until all of the following are true:

• Relay web and worker services are deployed and healthy
• the dtc-courses client/audience/sender scope exists in Relay
• a Relay API key exists in the main account secret used by CMP
• companion infrastructure PR DataTalksClub/aws-infra#20 is applied and ready for the coordinated ECS cutover

Merging the application change before the ECS environment change would intentionally disable email, because there is no DATAMAILER_* compatibility path.