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/server.go

@@ -81,9 +81,13 @@ var modelPostFormRoutes = []string{
 	"/v1/audio/transcriptions",
 	"/v1/images/edits",
 
-	// video generation (vLLM-Omni / OpenAI videos shape); extraction is
-	// content-type driven, so JSON bodies dispatch on these paths too
-	"/v1/videos",
+	// video generation: ONLY the blocking sync shape (vLLM-Omni
+	// /v1/videos/sync — the response body is the clip). The async
+	// OpenAI-style POST /v1/videos is deliberately NOT dispatched: its
+	// poll/download companions (GET /v1/videos/{id}[/content]) carry no
+	// model field to route by, so registering creation alone would start
+	// unretrievable upstream jobs. Extraction is content-type driven, so
+	// JSON bodies dispatch here too.
 	"/v1/videos/sync",
 }