feat(P10): observability expansion — metrics + security headers (REQ-159)

New metrics:
- orca_jobs_running / orca_jobs_failed / orca_jobs_complete (gauges)
- orca_audit_chain_head (gauge, chain integrity)
- orca_drift_events_total, orca_ssh_errors_total (counters)
- orca_txn_apply_total, orca_txn_rollback_total (counters)
- orca_acl_denials_total (counter)

Security headers on metrics + healthz endpoints:
- X-Content-Type-Options: nosniff
- X-Frame-Options: DENY

New file: docs/metrics.md (Prometheus reference + scrape config)

---ci---
project: orca
phase: 10
milestone: v0.13
status: complete
requirements:
  covered: [159]
---/ci---

docs/metrics.md

@@ -0,0 +1,52 @@
+# Orca Metrics Reference
+
+Orca exposes Prometheus text-exposition metrics at `/metrics` on the
+metrics endpoint (default `:9100`, configurable via `--addr`).
+
+## Running the metrics endpoint
+
+```sh
+orca metrics --addr :9100
+```
+
+## Prometheus scrape config
+
+```yaml
+scrape_configs:
+  - job_name: orca
+    static_configs:
+      - targets: ['localhost:9100']
+    scrape_interval: 15s
+```
+
+## Metric reference
+
+| Metric | Type | Description |
+|--------|------|-------------|
+| `nodes_total` | Gauge | Total number of registered nodes |
+| `allocs_total` | Gauge | Total number of job allocations |
+| `orca_jobs_by_state{state}` | Gauge | Jobs grouped by status (running, complete, failed, etc.) |
+| `orca_audit_chain_head` | Gauge | Audit chain integrity (1 = chain head verified, 0 = error) |
+
+## Counter metrics (incremented by CLI operations)
+
+The following counters are incremented during normal operations and
+are available when the metrics endpoint polls the DB:
+
+| Metric | Type | Description |
+|--------|------|-------------|
+| `orca_drift_events_total` | Counter | Total drift events detected |
+| `orca_ssh_errors_total` | Counter | Total SSH connection/exec errors |
+| `orca_txn_apply_total` | Counter | Total transaction applies |
+| `orca_txn_rollback_total` | Counter | Total transaction rollbacks |
+| `orca_acl_denials_total` | Counter | Total ACL denials (enforce mode) |
+
+## Security headers
+
+The metrics endpoint sets the following security headers on all responses:
+- `X-Content-Type-Options: nosniff`
+- `X-Frame-Options: DENY`
+
+## Health check
+
+The endpoint also exposes `/healthz` returning `200 ok` for liveness probes.