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>

k8s/prometheus/kustomization.yaml

@@ -14,3 +14,4 @@ configMapGenerator:
   - name: prometheus-rules
     files:
       - rules/host.rules.yml
+      - rules/ledgerline.rules.yml

k8s/prometheus/rules/ledgerline.rules.yml

@@ -0,0 +1,61 @@
+# Ledgerline alerting rules.
+#
+# ALERTING CONTRACT -- every alert in this directory (except scratch.*) must carry:
+#
+#   labels (machine-readable; Alertmanager routes and groups on these)
+#     severity   critical | warning | info
+#                critical = a human is needed now; warning = look today.
+#     team       who owns responding: platform | payments | risk | data
+#     service    which Ledgerline service, taken from the series' own label
+#                (aggregations must keep it: `by (service)`)
+#
+#   annotations (text for humans; never used for routing)
+#     summary      one line, no templating beyond the service name
+#     description  what is wrong, with the measured value and the threshold
+#     runbook_url  what to do about it (docs/runbooks/<kebab-alert-name>.md)
+#     dashboard    where to look first (the dashboard that explains the alert)
+#
+# observability/alerts/check_contract.py enforces this; run it before applying.
+#
+# `team` is static per alert rule, not per service: it says who answers this
+# *class* of alert. Per-service ownership would need a join against an
+# ownership metric or one rule per team.
+groups:
+  - name: ledgerline.rules
+    rules:
+      # All seven targets are scraped through one Traefik address, so a proxy
+      # failure takes every target down at once and fires seven alerts.
+      - alert: LedgerlineTargetDown
+        expr: up{job="ledgerline"} == 0
+        for: 2m
+        labels:
+          severity: critical
+          team: platform
+        annotations:
+          summary: "{{ $labels.service }} is not being scraped"
+          description: "Prometheus has failed to scrape {{ $labels.service }} for more than two minutes."
+          runbook_url: "https://registry.mohsen-hassani.com/mohsen_hassani/LedgerLine/src/branch/main/docs/runbooks/ledgerline-target-down.md"
+          dashboard: "https://grafana.vione.cloud/d/overview"
+
+      # 5xx only (a 404 is the client's mistake). The second condition keeps a
+      # service with almost no traffic from alerting on one failed request:
+      # below 0.05 req/s (about 15 requests per 5m) the ratio is noise. When
+      # there is no traffic at all the ratio is NaN and nothing fires.
+      - alert: LedgerlineHighErrorRatio
+        expr: |
+          (
+            sum by (service) (rate(http_server_request_duration_seconds_count{job="ledgerline",http_response_status_code=~"5.."}[5m]))
+            /
+            sum by (service) (rate(http_server_request_duration_seconds_count{job="ledgerline"}[5m]))
+          ) > 0.05
+          and
+          sum by (service) (rate(http_server_request_duration_seconds_count{job="ledgerline"}[5m])) > 0.05
+        for: 5m
+        labels:
+          severity: warning
+          team: payments
+        annotations:
+          summary: "{{ $labels.service }} is returning 5xx errors"
+          description: "{{ $labels.service }} has had a 5xx ratio of {{ $value | humanizePercentage }} for five minutes (threshold 5%)."
+          runbook_url: "https://registry.mohsen-hassani.com/mohsen_hassani/LedgerLine/src/branch/main/docs/runbooks/ledgerline-high-error-ratio.md"
+          dashboard: "https://grafana.vione.cloud/d/red-{{ $labels.service }}"