After a couple seconds on a detail page, a muted trailer plays behind the hero
(Netflix/Disney+ style) with mute/unmute + stop controls; the backdrop fades back
when stopped. Stops on navigate-away/modal-open (no orphaned audio).
Gated by a 'Autoplay trailers in the billboard' toggle in video Settings →
Detail Pages (default on). Backed by billboard_autoplay in video_settings, read
via a lightweight /api/video/prefs. Tests updated for the new config field.
2f3e4b128b