A pure read says the time to land: the median and the slowest hours from a pull's opening to its landing over the last 24 h

Flow work for #5930 (the KPI "time to clear").

Done when:

[ ] A new src/flow/lead_time.ts exports readLeadTime(db, now, publicOnly): { landed, medianHours, maxHours } for pulls merged into their default branch in the last 24 h, with both hours null when none landed.
[ ] It is one SQL read, with the same publicOnly rule as readVitals in src/flow/vitals.ts.
[ ] Tests: three landed pulls give the right median and max; a pull merged into a non-default branch is left out; one older than 24 h is left out; publicOnly hides private repositories.
[ ] Nothing reads it yet. Showing it waits for the vitals follow-ups.

Files: src/flow/lead_time.ts, a new test/flow-lead-time.test.ts.

A pure read says the time to land: the median and the slowest hours from a pull's opening to its landing over the last 24 h #6442