Mark Openings Seen

Clear the NEW badge on every opening this route is currently showing.

Includes EXPIRED (rebooked) rows, not only active ones. The detail screen renders a taken opening exactly like a live one — struck through, "booked at ..." — so opening the screen counts as seeing it just the same. An opening rebooked BEFORE the user opened the screen was already expired by the time this ran, so scoping to active rows left its seen_at NULL and stranded the "x new" badge on a row that could never be cleared — on the detail counter and on the alerts-list badge alike.

Re-stamps rows that are already seen but have REOPENED since, because that is what is_new now asks: record_openings revives a flickering start day on the SAME row and keeps its seen_at, so "seen once, ever" would strand a genuinely-new availability window as read (and the bell filter, which defaults to new-only, would then refuse to render the one bookable trip). Scoping the UPDATE to seen_at IS NULL did exactly that. The predicate below must stay the mirror of ItineraryOpening.is_new; if one moves, so does the other, or the badge never clears.

A user who never opens the screen never calls this, so a genuinely-unseen opening keeps its badge.