Speed up Dags list and dashboard queries on large DagRun tables

[yuseok89]

While reviewing #67242, we found that several existing UI queries are also slower than expected on large dag_run tables (verified with ~5M DagRuns). Adding new indexes would be a larger change (a migration), so this PR focuses on reusing the existing indexes. If there's a better direction, I'd appreciate your input.

Changes (per query)

• Dashboard historical_metrics: window now filters on the indexed run_after instead of the unindexed start_date/end_date.
• Dashboard dag_stats: latest run per Dag via correlated ORDER BY logical_date DESC LIMIT 1 instead of MAX(...) GROUP BY scanning every run.
• Dags list get_dags recent runs: top-N per Dag via ORDER BY run_after DESC LIMIT N instead of rank() over all runs.

Screenshots

Before

After

cc @pierrejeambrun

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Was generative AI tooling used to co-author this PR?

• Yes (please specify the tool below)
  • Opus 4.7

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