fix(ai-avatar): 标题分行/封面签名URL/渲染b_roll结构对齐后端

1. 标题 / 分行: 对齐智能剪辑 FrontendPreviewPlayer 的 split(/[/／]/) 逻辑，/ 和 ／ 都会分行
2. 封面签名URL: 私有桶 OSS upload 后必须 get_download_url 签名，否则前端无法加载
3. 渲染 b_roll_segments 结构: 前端 {asset: AssetItem} 转后端 {asset_url: string}，对齐 BRollSegment schema
4. #1821 对口型加速工单已关闭（收益有限，MediaKit GPU 排队是主要瓶颈）

apps/api/app/services/ai_avatar_cover_service.py

@@ -139,7 +139,11 @@ def persist_cover_to_oss(frame_url: str, *, job_id: str = "", prefix: str = "ai-
             content_type="image/jpeg",
         )
         logger.info("[数字人封面] 封面已转存 OSS: key=%s", cover_key)
-        return public_url or frame_url
+        # 私有桶：返回预签名 URL（前端才能加载）
+        if public_url:
+            signed = storage.get_download_url(cover_key, expires_seconds=86400)
+            return signed
+        return frame_url
     except Exception:
         logger.warning("[数字人封面] 封面转存 OSS 失败，返回原始 URL", exc_info=True)
         return frame_url

apps/web/src/pages/ai-avatar/AiAvatarPage.tsx

@@ -197,7 +197,15 @@ const AiAvatarPage: React.FC = () => {
       await submitRender({
         lipsync_job_id: state.lipsyncJob.id,
         script_id: state.script?.id,
-        b_roll_segments: state.bRollSegments as never,
+        b_roll_segments: state.bRollSegments.map((seg) => ({
+          script_segment_index: seg.script_segment_index,
+          asset_url: seg.asset.file_url || "",
+          mode: seg.mode,
+          start_time: seg.start_time,
+          end_time: seg.end_time,
+          pip_position: seg.pip_position,
+          pip_scale: seg.pip_scale,
+        })) as never,
         // 字段映射：build_title_drawtext_filter 真实口径 text/font_size/font_color/position/...
         title_config: buildTitleConfigPayload(state.titleConfig),
         // cover_config：智能封面 cover_url + 截帧 timestamp

apps/web/src/pages/ai-avatar/components/PanelLipsyncPreview.tsx

@@ -56,7 +56,6 @@ export function PanelLipsyncPreview({
         fontSize: `${(titleConfig.size || 36) * 0.55}px`, // 预览等比缩
         fontWeight: titleConfig.bold ? 700 : 400,
         fontStyle: titleConfig.italic ? "italic" : "normal",
-        whiteSpace: "pre-wrap",
         textAlign: "center",
         width: "90%",
         padding: "4px 8px",
@@ -142,7 +141,16 @@ export function PanelLipsyncPreview({
           {isDone && lipsyncJob?.output_video_url ? (
             <div style={{ position: "relative", width: "100%", height: "100%" }}>
               <video src={lipsyncJob.output_video_url} controls />
-              {titleOverlayStyle && <div style={titleOverlayStyle}>{titleConfig!.title}</div>}
+              {titleOverlayStyle && (
+                <div style={titleOverlayStyle}>
+                  {titleConfig!.title.split(/[/／]/).map((part, i) => (
+                    <span key={i}>
+                      {i > 0 && <br />}
+                      {part}
+                    </span>
+                  ))}
+                </div>
+              )}
             </div>
           ) : isGenerating ? (
             <div style={{ width: "80%", textAlign: "center", color: "#fff" }}>