Register signing secret for HMAC verification

Register an on-prem signing secret (opm_xxx) for HMAC verification.

Call once before sending telemetry (optional — the first batch auto-
registers). The secret is encrypted at rest. `cmp_` API keys are rejected;
mint an `opm_` credential in the dashboard under Settings -> On-Prem.

**Required Headers:**
- `X-API-Key`: Your Compresr API key

**Body:**
- `signing_secret`: On-prem signing secret (`opm_xxx`)
- `instance_id`: Optional instance identifier