The flow panel on /boards says the crew's build yield and review latency #6589

Done:

• [ ] Under the time-to-land line, the flow panel says the build yield over 24 h (runs finished, pushed, landed) and the median hours to first review, from src/flow/build_yield.ts and src/flow/review_latency.ts, public repositories alone for a non-admin.
• [ ] Test: a seeded database draws both lines.

Files: src/web/pages/flow-panel.ts, a test.