A milestone's page draws its issues closed per day as a §5 sparkline and says the date it is on pace to finish #5850

"Milestones you can watch move" (P8). Today the milestone page (src/web/pages/milestones.ts) shows where the milestone stands, not how fast it is moving.

Done when:

• [ ] A milestone's page shows the issues closed per day over the last 14 days, drawn with the design system's sparkline. Next to it, one line: on pace for <date> from the 7-day rate and the open count, no movement in 7 days, or done <date>.
• [ ] MCP milestone gives the same { closedPerDay, paceDate }.
• [ ] The read is one grouped query and a named Server-Timing phase, under 10 ms with 500 issues.
• [ ] Tests cover pace, a stalled milestone and a finished one.

Replaced by #6848. Parked twice: the reviews asked for one readPlan per load and for a test that reads the clock once. #6848 states both as done.