Prevent blank maintenance metadata from breaking monitoring

[masterfung]

What changed

• remove blank expectedDown: and expectedDegraded: lines from the scheduled-maintenance template
• tell operators to add those fields only when they have a real component value
• make repository validation reject blank impact metadata and retain safe examples

Root cause

The pinned Upptime monitor parses every colon-bearing metadata line with a split that assumes a non-empty value. The repository template instructed operators to leave optional impact values empty, so an active notice could crash the uptime workflow with Cannot read properties of undefined (reading 'trim').

Impact

Future maintenance notices can omit unused impact fields without breaking endpoint monitoring or leaving stale status data. Existing valid notices are unchanged.

Validation

• ruby scripts/validate.rb
• fresh live Uptime CI after correcting active metadata: passed
• fresh static-site build/publish: passed
• git diff --check