Mark Match Clicked

Record that the user tapped "Book" on a match (booking click-through).

Stamps clicked_at the first time only (idempotent — later taps don't overwrite). This is the last event we own before the user leaves for the provider's reservation site, so comparing clicked_at against booked_at reveals "tapped Book but the spot was already gone".

The mobile client fires this fire-and-forget right before opening the booking URL, so it must stay cheap and must never block that redirect — an unknown/foreign match_id is treated as a no-op (updated: 0) rather than an error.