Ingest telemetry batch from on-prem instance

Receive and store a batch of telemetry events from an on-prem Compresr instance.

**Required Headers:**
- `X-API-Key`: Compresr API key (authentication)
- `X-Compresr-Signature`: HMAC signature (integrity)

**Optional Headers:**
- `X-Instance-ID`: On-prem instance identifier

**First Request (auto-registration):**
Include `signing_secret` (`opm_xxx`) in the body - it will be registered
automatically. `cmp_` API keys are rejected; mint an `opm_` credential
in the dashboard under Settings -> On-Prem first.

**Signature Format:**
```
t=timestamp,v1=HMAC-SHA256(timestamp + "." + JSON(payload), signing_secret)
```

**Event Types:**
- `compression`: Counted for billing
- `error`: Logged only
- `startup`/`shutdown`: Operational

**Idempotency:**
Same batch_id within 24h returns success without reprocessing. A batch
with failed events stays replayable: retry it and already-billed events
are returned as status "duplicate" (never billed twice).