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).
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