adjustPostCount(of:by:on:)

func adjustPostCount(of fez: FriendlyFez, by delta: Int, on db: Database) async throws

Adds delta to the fez’s cached postCount with a single UPDATE ... SET post_count = post_count + delta, rather than writing back a value computed in Swift, so concurrent posts and deletes can’t overwrite each other’s change. The updated row stays locked until the surrounding transaction commits, which also serializes writers of the same fez. fez.postCount is set to the value the database now holds.