Make local Keycloak setup reproducible

[Subhransu-De]

What to build

Fix or document the local Keycloak setup so Docker Compose starts reproducibly. The Compose file references a realm export file that must exist or be generated.

Acceptance criteria

• Required Keycloak realm export file is committed or generated through a documented command.
• Docker Compose auth setup works from a fresh clone after setting required environment variables.
• README explains local test users, client configuration, and secrets.
• Compose remains clearly marked as local development infrastructure, not production Keycloak guidance.

Blocked by

None - can start immediately.