feat(api): On This Day shows one cover per burst; list a burst's frames #299

Parent

#296

What to build

On This Day shows each burst once, as the viewer's cover, marked with how many frames the viewer can see, and the API lists all of a burst's visible frames so the UI can page through them.

Acceptance criteria

• On This Day shows one item per burst: the viewer's cover, the earliest member that is visible to them (VisibleTo) and not hidden for them.
• The item carries burst { burstId, isCover, count }; count counts only members visible to the caller. Photos outside a burst carry no burst.
• A burst whose members are all hidden for the caller does not appear.
• A burst counts as starred, and as liked for DisplayWeight, when any visible member is.
• The cover's capture date decides the day: a burst across midnight appears on its cover's day only, and on the next day when every member before midnight is hidden for the caller.
• GET /api/v1/bursts/{id}/assets returns the burst's visible members, oldest first, as asset views through a new IAssetSelection read by IAssetViewReader; an unknown burst, another user's burst and a burst with no visible member all return 404.
• Tests: AssetViewReaderTests (SQL shape via ToQueryString(), data via InMemory), FlashbackYearsTests, FlashbackEndpointsTests. Mutation score 100% on the changed files.

First caller

On This Day (GET /api/v1/flashbacks/on-this-day) is the first caller of the collapse and the burst field; the UI (Epic 7) follows the burstId it hands out to GET /api/v1/bursts/{id}/assets, which ships in this PR so that id can be followed.

Design

Design note: #296 (comment) (section 6, option A)
Decision (maintainer, 2026-10-23): #296 (comment)