Add dashboards and alerts as code, mark Phase 6 done

Dashboards: observability/dashboards/generate.py (Grafana Foundation SDK)
writes k8s/grafana/dashboards/*.json -- a RED dashboard per HTTP service
and a system overview -- shipped through a file provider and
configMapGenerators, so `kubectl apply -k k8s/` rebuilds them. Verified by
deleting Grafana and its volume and re-applying. Adds tracesToMetrics to
the Tempo datasource.

Alerts: Prometheus rule files delivering to Alertmanager, under a
labels/annotations contract enforced by observability/alerts/check_contract.py,
with runbooks in docs/runbooks/.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

docs/runbooks/ledgerline-high-error-ratio.md

@@ -0,0 +1,27 @@
+# LedgerlineHighErrorRatio
+
+**Fires when:** a service's 5xx ratio is above 5% for 5 minutes, with more than
+0.05 requests/s (so it is not one failed request on an idle service).
+**Severity:** warning · **Team:** payments
+
+## What it means
+
+A meaningful share of the requests this service serves are failing with a server
+error. 4xx responses (for example a 404) are deliberately not counted.
+
+## First five minutes
+
+1. **Look at the `RED: <service>` dashboard** (linked in the alert). Is the error
+   ratio still elevated, or already back to normal? Did the request rate or the
+   latency change at the same time?
+2. **Find a failing request.** In Tempo, search for the service with an error
+   status, or in Loki: `{service_name="<service>"} | json | level="ERROR"`.
+3. **Follow one trace.** The `trace_id` in the log line opens the whole request
+   across services. Look for the first span in the trace with an error status.
+4. **Is it only this service?** The System overview's service graph shows whether
+   its callers or callees are failing as well.
+
+## Escalate when
+
+The ratio is still rising after ten minutes, or the failing requests are on the
+critical path (transactions), not a side route.