fix: select existing tenants by name in the operator portal

Assistant: codex
Assistant-Model: gpt-6-astra
Assistant-Session: 01a07ff8-19d0-7820-b4d0-1353833cb7fc

src/user_engine/ports.py

@@ -282,6 +282,9 @@ class UserEngineStore(Protocol):
     def memberships_for_tenant(self, tenant: str) -> tuple[Membership, ...]:
         """Return memberships scoped to a tenant."""
 
+    def membership_tenants(self) -> tuple[str, ...]:
+        """Return distinct tenant identifiers referenced by membership records."""
+
     def save_application(self, application: Application) -> None:
         """Create or replace an application registration."""