fix(upstream): vivid 协议的 quality 原样透传分辨率档位

vivid 的 quality 就是模型自己的档位字符串——图像 1K/2K/4K、视频
720p/1080p。之前映射成 OpenAI 的 low/medium/high 导致视频上游收到
"1K" 报 4001 "该模型支持 720p"。现在原样透传,空值才默认 1K。

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Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

backend/internal/service/v1.go

@@ -2475,11 +2475,12 @@ func upstreamVideoSize(aspectRatio, resolution string) string {
 }
 
 // vividQuality maps a resolution tier to the quality string a Vivid-protocol
-// upstream expects ("1K"/"2K"/"4K"); empty → "1K" (required field).
+// upstream expects. Vivid passes the tier through verbatim — "1K"/"2K"/"4K"
+// for images, "720p"/"1080p"/"540p" for videos — so don't re-map to the OpenAI
+// enum; only default a blank to "1K" (the field is required).
 func vividQuality(resolution string) string {
-	switch strings.ToUpper(strings.TrimSpace(resolution)) {
-	case "2K", "4K":
-		return strings.ToUpper(strings.TrimSpace(resolution))
+	if q := strings.TrimSpace(resolution); q != "" {
+		return q
 	}
 	return "1K"
 }