scan_timelapse's Strategy 2 matched filename timestamps against both
archive.started_at and archive.completed_at across seven hypothesised tz
offsets. The filename is always print-START time, so the end-time branch
was a semantic mistake — and the dense offset set [0, +-1, +-7, +-8]
let an unrelated video coincidentally land within minutes of any later
archive at some offset.
Extract Strategy 2 into _match_timelapse_by_timestamp(): compare only
against start time, and refuse to auto-pick when the next-best different
video is within a 15-minute ambiguity margin. The route then returns
available_files and the frontend's existing manual-selection dialog
takes over — which is the fallback the reporter explicitly asked for.
Surfaces in LAN-Only mode where the printer can't reach NTP and its
clock drifts (e.g. P2S filenames in CST while server is in UTC, the
8h offset that exposed this bug).
