List stored BYOK LLM keys

Scope: llm_keys (granted to new live keys by default). Requires a live API key; sandbox keys cannot inspect the production credential store. Returns non-secret metadata only: provider, sha256-prefix fingerprint, timestamps. The key itself is never returned.

A key-store outage returns 503 DB_UNAVAILABLE with Retry-After, not a bare 500, so a transient failure is distinguishable from a permanent one and is safe to retry.