docs: Document automatic FK discovery in PostgreSQL catalog connector

The PostgreSQL Catalog Connector now discovers foreign key relationships
during catalog refresh and surfaces them via the table_schema tool and
FlightSQL GetTables.

Author: claudespice

website/docs/components/catalogs/postgres.md

@@ -100,6 +100,15 @@ catalogs:
       pg_connection_string: postgresql://${secrets:REDSHIFT_USER}:${secrets:REDSHIFT_PASS}@my-cluster.abc123.us-east-1.redshift.amazonaws.com:5439/my_database?sslmode=require
 ```
 
+## Foreign Key Discovery
+
+The PostgreSQL Catalog Connector automatically discovers foreign key relationships by querying `information_schema.referential_constraints` and `key_column_usage` during catalog refresh. Discovered FK metadata is attached to each table's Arrow schema and surfaces through:
+
+- The `table_schema` tool — agents can use FK relationships to infer join paths between tables
+- FlightSQL `GetTables` — programmatic clients receive FK metadata in the schema response
+
+No configuration is required. If FK discovery fails for a schema (e.g., due to insufficient permissions on `information_schema`), tables are still registered without FK metadata and a warning is logged.
+
 ## Secrets
 
 Spice integrates with multiple secret stores to help manage sensitive data securely. For detailed information on supported secret stores, refer to the [secret stores documentation](../secret-stores). Additionally, learn how to use referenced secrets in component parameters by visiting the [using referenced secrets guide](../secret-stores#using-secrets).