A map's ask-to-live medians come from one batched read: `asktolive` under 50 ms for trevato/forge #6175

Measured 2026-10-26: GET /api/v1/repos/trevato/forge/map takes 1.6 s, and asktolive;dur=1163.7 is most of it. MCP map and the map page call the same askToLiveFor (src/web/ask_to_live.ts).

Done when:

• [ ] askToLiveFor reads every merged pull of the window and its issue's opening time in a fixed number of queries, not one per pull or per pillar.
• [ ] Its answer is unchanged: a test compares the median and slowest per pillar against the current implementation on a fixture.
• [ ] A test with 400 merged pulls across 8 pillars over 7 days asserts it runs under 50 ms.

Files: src/web/ask_to_live.ts, test/ask-to-live*.test.ts. Risk: a sealed or private issue must stay out of the medians for a viewer who cannot see it. Keep the user filter.