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/grafana/dashboards-provider.yml

@@ -0,0 +1,13 @@
+apiVersion: 1
+
+providers:
+  - name: ledgerline
+    orgId: 1
+    type: file
+    folder: Ledgerline
+    options:
+      path: /etc/grafana/dashboards/
+    allowUiUpdates: false
+    disableDeletion: false
+    updateIntervalSeconds: 15
+    foldersFromFilesStructure: false