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/datasources.yml

@@ -38,6 +38,22 @@ datasources:
         filterByTraceID: true
         spanStartTimeShift: -1m
         spanEndTimeShift: 1m
+        # QUESTION (answer in your own words; verify by reading the LogQL that
+        # "Logs for this span" generates): why is `filterByTraceID: true` enough
+        # with no `tags` mapping here, and when would it not be?
+      tracesToMetrics:
+        datasourceUid: prometheus
+        # Wider than the span: rate() needs a few scrape intervals of data.
+        spanStartTimeShift: -2m
+        spanEndTimeShift: 2m
+        # span attribute -> Prometheus label on traces_spanmetrics_*
+        tags:
+          - key: service.name
+            value: service
+        queries:
+          # `$$` is the provisioning-file escape for a literal `$`.
+          - name: Request rate
+            query: sum(rate(traces_spanmetrics_calls_total{$$__tags}[5m]))
       serviceMap:
         datasourceUid: prometheus
       nodeGraph: