feat: add Grafana alerting rules and documentation (refs #111)

Configure Grafana Unified Alerting with file-based provisioned alert
rules, contact points, and notification policies. Add stable UID to
Loki datasource for alert rule references. Update LOGGING.md with
dashboard descriptions, alerting rules table, and LogQL query reference.

Alert rules: Error Rate Spike (critical), Container Silence for
backend/postgres/redis (warning), 5xx Response Spike (critical).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

config/grafana/alerting/notification-policies.yml

@@ -0,0 +1,11 @@
+apiVersion: 1
+
+policies:
+  - orgId: 1
+    receiver: mvp-default
+    group_by:
+      - alertname
+      - severity
+    group_wait: 30s
+    group_interval: 5m
+    repeat_interval: 4h