Server-Timing on / names each part of readKpis: landed, live, checks, builders, spend, reach, backup #7626

Part of #7494. In production, kpis costs 4.3–4.6 s and nobody knows which part. The last try guessed, added a migration, and went red.

Done when:

• [ ] readKpis (src/flow/kpis.ts) accepts an optional time(name, fn) callback. The home passes one that adds a Server-Timing entry for each part: kpis.landed, kpis.live, kpis.checks, kpis.builders, kpis.spend, kpis.reach, kpis.backup.
• [ ] Other callers of readKpis are unchanged.
• [ ] A test asks for / as an admin and finds the seven kpis.* names in the Server-Timing header.
• [ ] No query changes, no migrations.

Files: src/flow/kpis.ts, src/web/studio/home.ts, one test. After it lands: the steward reads the header in production, and the slowest part is the next issue's target.