Configure Grafana alerting for error spikes

[GitAddRemote]

Summary

Set up Grafana alert rules to notify on error spikes and other critical log conditions once Grafana Loki is running.

Acceptance criteria

• Alert fires when error-level logs (pino level >= 50) exceed 10 occurrences in a 5-minute window
• Alert is delivered to a configured notification channel (email or webhook)
• Alert rules are defined as code (Grafana provisioning YAML) so they are version-controlled and survive container restarts
• A silencing window can be applied during planned deployments

Implementation notes

• Use Grafana's built-in alerting (not the legacy version)
• Define alert rules in infra/grafana/provisioning/alerting/ so they are provisioned on container start
• Configure a contact point in infra/grafana/provisioning/alerting/contact-points.yaml
• Document the alert setup in infra/docs/logging.md

Prerequisites

• Grafana Loki setup complete (see Loki issue)