Revoke Retriever API Key

Revoke a retriever-scoped API key.

**Effect:**
- Key status is set to REVOKED
- Key can no longer be used for authentication
- Revocation is immediate (no grace period)
- Auth cache is invalidated immediately
- Cannot be undone (create a new key if needed)

**Audit:**
- Revocation is logged in the retriever's audit trail
- Includes actor user ID and timestamp