feat(api): add video generation endpoint

Add POST /sdapi/v1/video plus GET /sdapi/v1/video/models and
GET /sdapi/v1/video/file. The generation body is extracted from the
gradio handler into a keyword-only core, video_run.run, which returns a
structured result and raises typed errors; the positional generate
signature is unchanged and now adapts to the core. Omitting engine and
model drives the currently loaded checkpoint when it is video-capable,
which covers models loaded from local folders without a registry entry.

- registry helpers in models_def (find, engines, pipeline_classes,
  workflow_for_class); validate_pipeline reuses the shared class set
- modular pipes stamp their workflow so out-of-registry loads dispatch
  onto the modular branch
- disk switches (mp4_*) and wire switches (send_*) are independent;
  artifacts above the base64 cap fall back to path plus the file route,
  which is jailed to the video output directory and serves video/mp4
  with range support
- always-on video scripts get bootstrapped default args, matching the
  txt2img handler; missing bootstrap raised a TypeError per frame
- checkpoint overrides are rejected with a pointer to the checkpoint
  endpoint; unknown engine, model and sampler names return 404 with the
  valid choices
- cli/api-video.py client, test/test-video-api.py suite and a
  full-test.sh entry; video mimetypes registered; rate-limit cost set
- remove the unreferenced video_ui.run_video dispatcher

modules/api/validate.py

@@ -12,6 +12,7 @@ request_cost = {
     "/sdapi/v1/txt2img": 5,
     "/sdapi/v1/img2img": 5,
     "/sdapi/v1/control": 5,
+    "/sdapi/v1/video": 5,
 }
 log_cost = {
     "/.well-known/appspecific/com.chrome.devtools.json": -1,