fix: scope video dispatch to /v1/videos/sync + capture mask

Review findings: the async POST /v1/videos leg is deliberately NOT
dispatched — its poll/download companions carry no model field to route
by, so registering creation alone would start unretrievable upstream
jobs (a stray OpenAI-videos client now gets a clean 404). And
/v1/videos/sync joins captureFieldsByPath (headers only, both
directions) so enabling captures doesn't buffer conditioning frames or
whole clips through cbor+zstd.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01AXQxVhXBw8PwFAtsVrXSmj

internal/server/captures.go

@@ -37,7 +37,8 @@ const (
 )
 
 // captureFieldsByPath overrides the default capture mask for routes carrying
-// large binary payloads (audio/image) where storing the full body is wasteful.
+// large binary payloads (audio/image/video) where storing the full body is
+// wasteful.
 var captureFieldsByPath = map[string]captureFields{
 	"/v1/audio/speech":         captureReqAll | captureRespHeaders,
 	"/v1/audio/voices":         captureReqHeaders | captureRespAll,
@@ -46,6 +47,9 @@ var captureFieldsByPath = map[string]captureFields{
 	"/v1/images/edits":         captureReqHeaders | captureRespHeaders,
 	"/sdapi/v1/txt2img":        captureReqAll | captureRespHeaders,
 	"/sdapi/v1/img2img":        captureReqHeaders | captureRespHeaders,
+	// video: the request multipart can carry a conditioning frame and the
+	// response body IS the clip — headers only, both directions.
+	"/v1/videos/sync": captureReqHeaders | captureRespHeaders,
 }
 
 // captureFieldsFor returns the capture mask for a request path. Unlisted routes