Export/share report as Markdown

[CodeBuildder]

Context

Markdown export of the daily report card. Parent: #42.

Acceptance Criteria

• GET /reports/daily/{date}/export returns Markdown-formatted report
• Markdown includes all metrics, trendline summaries, and top incidents
• Export is idempotent (same output for same date regardless of call time)
• Report renders correctly in GitHub Markdown preview

Dependencies

Blocked by: none | Blocks: none

Notes

No model tier.