Expose per-model video prompt_extend as a toggle (it was silently on until the Sept 11 fix)

What I'm asking for: a toggle for video prompt extension, per model, default off, with a visible way to see what was actually sent. Same shape as the existing image enhance_prompt behavior: opt-in, fails open to the original prompt.

Why: until the September changelog, Wan 3.0 was silently applying upstream prompt extension on top of Venice's own enhancer. That upstream layer is what was adding camera moves, motion beats and audio atmosphere. It's now off and prompt_extend is honored per-model — but nothing in the UI or API exposes it, so there's no way to turn it back on.

The result is that the wand on a video prompt returns my text essentially unchanged. The Venice-side rewriter is image-templated (composition, lighting, mood) and on video it has nothing to layer on top of, so it echoes. Before the fix, the same prompt produced shot language, movement and ambience.

Evidence: the Sept 2026 changelog, Fixes, "Wan Prompt Extension Fix — Wan 3.0 and Wan 3.0 Prime video generation no longer silently applies upstream prompt enhancement, and the model's prompt_extend setting is now honored." Since then, Sept and Oct have 27 and 128 image/video/audio feature entries between them and not one touches video prompt enhancement — every enhancer change is image or edit (mask-aware, aspect-aware, edit wand, variant-aware fix, filmstrip grouping).

What it should look like:

A switch in Video Studio settings, per model, remembered per model the way other video settings are.
Off by default, so people who hand-write structured prompts keep full control.
A visible preview or expandable "enhanced prompt" on the draft card, so it's auditable — this is the part that would have made the silent version tolerable in the first place.
Same field on /video/queue as prompt_extend (bool), defaulting false, so API users aren't stuck either.

Why not just leave it off: for Wan 3.0 it's genuinely good at exactly the things that are tedious to hand-write — timestamped beats, named camera moves, room tone. For a rough idea it's faster than writing the structure myself. But it should be a choice, not a hidden default in either direction.