simplify production database configuration

.env.example

@@ -41,11 +41,10 @@ ZHINIAN_WORKER_REQUEST_TIMEOUT_MS=120000
 # Production must explicitly select postgres; it never falls back to container-local JSON.
 ZHINIAN_DATA_BACKEND=local
 DATABASE_URL=
-# Migration runner only: PostgreSQL role used by the Web DATABASE_URL.
+# Migration runner only: PostgreSQL role used by the Go API DATABASE_URL.
 DATABASE_APP_ROLE=
-DATABASE_SSL_MODE=disable
-# For RDS SSL, set verify-full and mount the downloaded CA certificate at this path.
-DATABASE_CA_CERT_PATH=
+# Optional URL query for an explicit RDS CA, for example:
+# postgresql://user:password@rds-host:5432/app?sslmode=verify-full&sslrootcert=/etc/zhinian/rds/ca.pem
 DATABASE_POOL_MAX=10
 DATABASE_IDLE_TIMEOUT_MS=30000
 DATABASE_CONNECTION_TIMEOUT_MS=5000