feat(demos): add multi-tenant with shared schema demo

[zeevdr]

Summary

• Demonstrates the core SaaS multi-tenancy pattern: one schema shared by all tenants, with each tenant managing its own config values completely independently.
• Provides a concrete, runnable example that answers "how do I onboard a second customer without duplicating schema definitions?"
• Completes the demo content requested in the parent issue for multi-tenant scenarios.

Test plan

• ./run.sh completes without errors (infrastructure up, schema seeded, both tenants provisioned)
• curl http://localhost:8080/v1/config/tenant1/snapshot returns USD currency and 8% tax rate
• curl http://localhost:8080/v1/config/tenant2/snapshot returns EUR currency and 20% tax rate
• Runtime override on tenant1 does not affect tenant2's values (isolation check in run.sh)
• Re-running seeds is idempotent (no duplicate versions created)
• docker compose down -v tears down cleanly
• Admin panel at http://localhost:3000 loads in multi-tenant mode

Refs opendecree/decree#30