Set up disk usage alerts in Lightsail

[aaylward]

Set up alerts in AWS Lightsail so we get notified when disk usage is high (e.g. approaching full), to avoid surprise disk-full situations.

Context: Services like one_d4 (H2 file DB in one_d4_data volume) and others can grow disk usage over time; there is no app-level or volume cap today (see one_d4 roadmap disk-cap item). Alerts give operators a chance to clean up or scale before the instance runs out of space.

Suggested scope:

• Define threshold(s), e.g. alert when disk usage > 80% (warning) and > 90% (critical).
• Configure Lightsail alarms/notifications for the instance(s) that run the consolidated stack (or per-instance if multiple).
• Document where alerts are sent (email, Slack, etc.) and how to respond (e.g. which volumes/paths to check, link to disk-cap roadmap if relevant).