Land "Since you were here" from pull #6684's branch, with its duplicate `pull_merged` index taken out #6711

Replaces #5804. Pull #6684, branch scout/5804-since-you-were-here-means-since-you-last, was reviewed three times and holds every ask but the last. Start from that branch, keep its history, and finish it in one new pull.

Done when:

• [ ] The migration no longer creates pull_merged, because that index already exists in src/core/migrations/20261022120000-flow-indexes.ts. The digest query still uses that index; check with EXPLAIN QUERY PLAN in a test or by hand, and say which in the pull.
• [ ] Everything the reviews accepted stays as it is:

- one home_visit row per user, written at most once a minute; - the window runs from the last visit, clamped to 1 h–7 d, with 24 h on a first visit; - the heading says Since you were here · 2 days; - only a real navigation records a visit: no prefetch, preview, or a Sec-Fetch-Dest other than document; - the terminal dashboard shows the same lines and never moves the window; - the section is drawn with listCard and row().

• [ ] The tests assert the visit phase is present on / and do not time it. The 3-hour, 10-day, throttle, first-visit and prefetch tests pass.

Files: src/web/studio/since.ts, src/web/studio/home.ts, src/tui/screens/dashboard.ts, test/home-since.test.ts