The Surprise rail drew a random 20+20 sample including already-watched titles. Its point is
to surface something to WATCH, so exclude seen titles (a watched movie, a fully-watched show
— a partially-watched or never-started show still counts as unseen) and cap at 3 movies + 3
shows (6 total), interleaved.
Verified live: /surprise returns 6 items, 3+3, none watched.
Gate: backend ruff clean, 12 discover tests (+ a watched-exclusion/cap assertion).