leaderboard: open reports in new tab

leaderboard: drop silent fallback; boot resilient on Hub read failure

leaderboard: format submitted_at as `YYYY-MM-DD HH:MM UTC`; lock tables read-only

leaderboard+app: combined CSV download with validation_status discriminator

app: render reports inline via iframe srcdoc (private-Space safe)

app+leaderboard: link submission_name to a Space-side report proxy

app+leaderboard: detail panel polish (rename link columns, fix broken report links)

leaderboard: markdown link columns for agent_url, submission, report

leaderboard: add load_leaderboard_split() for two-tier viewer