docs(architecture): observability alerting + admin-alert worker

§11 gains the alerting layer: Grafana infra rules (scrape-down, edge error-rate/p99, host
mem/disk/cpu, postgres connections, TLS cert < 20d via blackbox), noDataState=OK, and the
backend admin-alert worker (coalesced email on new feedback / complaints).

docs/ARCHITECTURE.md

@@ -1003,6 +1003,19 @@ browser), so an open in-app session reflects it at once.
   both are surfaced on the **Scrabble — Resources** Grafana dashboard, which captures the
   stress-run resource profile. (`docker_stats` replaced cAdvisor, which on the contour
   host resolved only the root cgroup — a separate-XFS `/var/lib/docker`.)
+- **Alerting.** Grafana emails infra alerts through the shared relay (its own SMTP on the
+  STARTTLS port) to `SERVICE_EMAIL`, from provisioned rules
+  (`deploy/grafana/provisioning/alerting/`): a scrape-target down, the gateway's
+  internal-error rate and p99 latency (`edge_request_*`), host memory/disk/CPU
+  (node_exporter), Postgres connection saturation (postgres_exporter), and **TLS certificate
+  expiry < 20 days** — a Caddy ACME-renewal-failure signal from a **`blackbox_exporter`**
+  probe of the edge caddy (`probe_ssl_earliest_cert_expiry`). Every rule is `noDataState=OK`,
+  so an absent metric never false-alerts — notably the cert probe, which has no TLS target on
+  the HTTP-only contour caddy. Separately, the backend's **admin-alert worker**
+  (`internal/adminalert`, started from `main`) emails the operator (`ADMIN_EMAIL`, from a
+  distinct `SMTP_RELAY_ADMIN_FROM`) when new player feedback or word complaints arrive,
+  **coalescing** a burst into one digest per interval; both recipients may be several
+  comma-separated addresses. Both paths are inert unless configured.
 - Per-request server-side timing via gin middleware from day one (the access log
   carries method, route, status, latency and the active trace id). A
   client-measured RTT piggybacked on the next request is a later enhancement.