Multi-tenant demo: shared schemas, isolated config

[zeevdr]

Demonstrate multi-tenancy: one decree server, shared schemas, per-tenant config isolation.

• docker-compose.yml
• Setup script (create tenants, shared schema)
• Walkthrough showing isolation
• test.sh for CI validation

Part of opendecree/decree#30.

[github-actions]

This issue has been inactive for 60 days and is being marked stale. It will be closed in 14 days unless there is new activity. Add the keep label to opt out of future sweeps.