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/prometheus/blackbox.yml

@@ -0,0 +1,13 @@
+# blackbox_exporter config — HTTP probes for uptime monitoring (Phase 4 §17). Probes the
+# /healthz endpoints added to Platform API and Gateway this phase, plus the Provisioner's
+# existing /health (Phase 1). Prometheus scrapes blackbox_exporter with these probe targets
+# (see prometheus.yml.j2's blackbox-* jobs — add them there once blackbox_exporter is deployed;
+# left as a documented follow-up wiring step rather than duplicated here).
+
+modules:
+  http_2xx:
+    prober: http
+    timeout: 5s
+    http:
+      valid_status_codes: [200]
+      method: GET