chore: streamline configuration (add config objects to commons to reference in services)

docs: add configuration reference page

Centralises repeated configuration by moving shared oblx.* property defaults (Keycloak URL/realm, OpenFGA URL, ClickHouse host/port) into commons/definitions so individual services can reference them via property placeholders rather than redeclaring the same values.

Test configuration across all services is simplified and a bug which arose in StreamingResourceTest is fixed (filter SSE messages by source label client-side, making it robust against stale Kafka messages from previous runs on the shared broker).