feat(deploy): Grafana infra alerts + blackbox cert probe + admin-alert wiring

Grafana: GF_SMTP from the shared relay (STARTTLS host:port) + alerting provisioning
(contact point → SERVICE_EMAIL, route-all policy, and rules for scrape-target down,
gateway internal-error rate + p99 latency, host mem/disk/cpu, postgres connections, and
TLS cert < 20 days). All rules noDataState=OK so an absent metric never false-alerts.
blackbox_exporter probes the edge caddy's TLS (probe_ssl_earliest_cert_expiry) — effective
on prod (caddy terminates TLS; contour caddy is HTTP-only so the metric is absent).
Wires the new env through compose (backend admin From/To, Grafana SMTP), ci.yaml (TEST_),
prod-deploy (PROD_ + env.sh), .env.example and the README var table.

deploy/grafana/provisioning/alerting/policies.yaml

@@ -0,0 +1,10 @@
+# Notification policy: every alert routes to the operator email, grouped so a burst is one
+# message, with a 4-hour re-notify while still firing.
+apiVersion: 1
+policies:
+  - orgId: 1
+    receiver: ops-email
+    group_by: ['alertname']
+    group_wait: 30s
+    group_interval: 5m
+    repeat_interval: 4h