The migrate page and a migration job's page name their Server-Timing phases: session, each read, render #3504

Intent: this page says where its time goes, like the milestones and usage pages do (speed is first class; the sweep test #3387 will hold every page to it).

Done:

• [ ] migratePage (GET /migrate) and migrationJobPage (GET /migrate/jobs/:job) answers with a Server-Timing header naming session, one phase per data read the page makes (named for what it reads), and render, using notePhase/timePhase from src/core/route_timing.ts exactly as src/web/pages/milestones.ts does.
• [ ] test/migrate-timing.test.ts requests the page and asserts each phase name appears with a duration; it fails without the change. It asserts names, never a wall-clock time.
• [ ] The page's body, status codes and caching are unchanged.

Files: src/web/pages/migrate.ts, test/migrate-timing.test.ts. Touch no other page. Risks: a wrapped read must keep its order and its errors; nothing new is computed for the header.