feat(observability): Prometheus/Alertmanager/Grafana/Loki stack

Self-hosted, no paid SaaS. Host/container/Postgres metrics, uptime probes
against the new /healthz endpoints, alert rules split critical (pages) vs
warning (informational), one starter Grafana dashboard. Deliberately
separate from the application's instructor learning analytics (Phase 3B) -
different audience, different data.

Validated: promtool check config (valid), promtool check rules (11 rules,
valid), amtool check-config (1 inhibit rule, 2 receivers, valid).

infra/monitoring/grafana/provisioning/dashboards/dashboards.yml

@@ -0,0 +1,7 @@
+apiVersion: 1
+providers:
+  - name: RootNode
+    folder: RootNode
+    type: file
+    options:
+      path: /var/lib/grafana/dashboards

infra/monitoring/grafana/provisioning/datasources/datasources.yml

@@ -0,0 +1,11 @@
+apiVersion: 1
+datasources:
+  - name: Prometheus
+    type: prometheus
+    access: proxy
+    url: http://prometheus:9090
+    isDefault: true
+  - name: Loki
+    type: loki
+    access: proxy
+    url: http://loki:3100