fix: address PostgreSQL registry review

internal/postgresql/registry/migrations/001_create_registry.sql

@@ -0,0 +1,25 @@
+CREATE SCHEMA postgresql_tenant_operator;
+
+CREATE TABLE postgresql_tenant_operator.tenant_ownership (
+    instance_uid text NOT NULL,
+    tenant_uid text NOT NULL,
+    tenant_namespace text NOT NULL,
+    tenant_name text NOT NULL,
+    database_name text NOT NULL,
+    role_name text NOT NULL,
+    credential_path text NOT NULL,
+    managed boolean NOT NULL DEFAULT true,
+    created_at timestamptz NOT NULL DEFAULT clock_timestamp(),
+    updated_at timestamptz NOT NULL DEFAULT clock_timestamp(),
+    retained_at timestamptz,
+    PRIMARY KEY (instance_uid, tenant_uid),
+    UNIQUE (instance_uid, tenant_namespace, tenant_name),
+    UNIQUE (instance_uid, database_name),
+    UNIQUE (instance_uid, role_name),
+    UNIQUE (credential_path),
+    CHECK (managed OR retained_at IS NOT NULL)
+);
+
+---- create above / drop below ----
+
+DROP SCHEMA postgresql_tenant_operator CASCADE;