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/helpers.py

@@ -1,4 +1,5 @@
 import io
+import os
 import base64
 from PIL import Image, PngImagePlugin
 import piexif
@@ -90,6 +91,24 @@ def encode_pil_to_base64(image):
     return b64
 
 
+MAX_B64_BYTES = 256 * 1024 * 1024 # base64 expands ~4/3 and the response is built in memory; larger artifacts are fetched by path instead
+
+
+def encode_file_to_base64(fn: str, max_bytes: int = MAX_B64_BYTES) -> str | None:
+    try:
+        if fn is None or not os.path.isfile(fn):
+            return None
+        size = os.path.getsize(fn)
+        if size > max_bytes:
+            log.warning(f'API cannot encode file: fn="{fn}" size={size} max={max_bytes}')
+            return None
+        with open(fn, 'rb') as f:
+            return base64.b64encode(f.read()).decode('ascii')
+    except Exception as e:
+        log.warning(f'API cannot encode file: fn="{fn}" {e}')
+        return None
+
+
 def upscaler_to_index(name: str):
     try:
         return [x.name.lower() for x in shared.sd_upscalers].index(name.lower())