Export character relationship scores

Export final relationship score meters for one app-owned character. This is the app-dashboard companion to ``GET /v1/sessions/{id}/relationship-scores``. It returns one row per app-owned session for the character, keyed by ``session_id`` and ``external_user_ref``. It uses the same app opt-in as the session endpoint: until ``expose_relational_numerics`` is true, the response is redacted and empty. Raw extractor deltas, patch history, signal events, critic reasoning, and prompt text are never exposed.