A pure read says, per repository, how many approved pulls wait for the merge queue and how long the oldest has waited

Part of #5930: the approved pile per repository, not only forge-wide.

Done:

[ ] New src/flow/approved_wait.ts exports approvedWait(rows, now), taking { repo, number, approvedAt }[] and returning { repo, waiting, oldestHours, oldest }[], longest wait first.
[ ] Hours are rounded to one decimal; an approval in the future reads as 0.
[ ] Tests: one repository, several, ordering, the rounding.

Files: src/flow/approved_wait.ts, test/flow-approved-wait.test.ts.

Before pushing: bun x tsc --noEmit and the new test.

A pure read says, per repository, how many approved pulls wait for the merge queue and how long the oldest has waited #6552