fix(P0): 修复缩略图生成失败时临时文件泄漏 + 素材库视频缩略图生成

- ingest流程中添加视频缩略图生成（使用15%处抽帧）
- 缩略图生成失败不影响主流程，仅打warning日志
- 修复extract_first_frame失败时临时文件未清理的bug
- 修复素材库视频全部显示渐变占位图的问题

Issue: #577

apps/worker/video_processing/thumbnail_generator.py

@@ -38,53 +38,36 @@ def extract_first_frame(
     """
     from video_processing.ffmpeg_utils import FFMPEG_BIN, probe_duration, run_ffmpeg
 
+    _is_temp_output = False
     if output_path is None:
         tmp = tempfile.NamedTemporaryFile(suffix=".jpg", delete=False)
         tmp.close()
         output_path = tmp.name
-
-    # 计算抽帧时间点：取视频时长 * seek_ratio，最少 min_seek_seconds 秒
-    try:
-        duration = probe_duration(video_path)
-        seek_time = max(min_seek_seconds, duration * seek_ratio)
-    except Exception:
-        # probe 失败时 fallback 到第1秒
-        seek_time = min_seek_seconds
-
-    # 格式化为 HH:MM:SS.xx
-    seek_str = _format_seek_time(seek_time)
-
-    # -ss 放在 -i 前面（input seeking，更快但精度稍低，缩略图够用）
-    # -vframes 1 只取一帧
-    # -q:v 2 jpeg 高质量
-    scale_filter = f"scale={width}:{height}:force_original_aspect_ratio=decrease"
-    cmd = [
-        FFMPEG_BIN,
-        "-y",
-        "-ss",
-        seek_str,
-        "-i",
-        video_path,
-        "-vframes",
-        "1",
-        "-vf",
-        scale_filter,
-        "-q:v",
-        "2",
-        output_path,
-    ]
+        _is_temp_output = True
 
     try:
-        run_ffmpeg(cmd, capture_output=True, timeout=timeout)
-    except Exception:
-        # 失败时退回到第0帧兜底
-        cmd2 = [
+        # 计算抽帧时间点：取视频时长 * seek_ratio，最少 min_seek_seconds 秒
+        try:
+            duration = probe_duration(video_path)
+            seek_time = max(min_seek_seconds, duration * seek_ratio)
+        except Exception:
+            # probe 失败时 fallback 到第1秒
+            seek_time = min_seek_seconds
+
+        # 格式化为 HH:MM:SS.xx
+        seek_str = _format_seek_time(seek_time)
+
+        # -ss 放在 -i 前面（input seeking，更快但精度稍低，缩略图够用）
+        # -vframes 1 只取一帧
+        # -q:v 2 jpeg 高质量
+        scale_filter = f"scale={width}:{height}:force_original_aspect_ratio=decrease"
+        cmd = [
             FFMPEG_BIN,
             "-y",
+            "-ss",
+            seek_str,
             "-i",
             video_path,
-            "-ss",
-            "00:00:00",
             "-vframes",
             "1",
             "-vf",
@@ -93,12 +76,40 @@ def extract_first_frame(
             "2",
             output_path,
         ]
-        run_ffmpeg(cmd2, capture_output=True, timeout=timeout)
 
-    if not Path(output_path).exists() or Path(output_path).stat().st_size == 0:
-        raise RuntimeError(f"Thumbnail generation failed: {output_path}")
+        try:
+            run_ffmpeg(cmd, capture_output=True, timeout=timeout)
+        except Exception:
+            # 失败时退回到第0帧兜底
+            cmd2 = [
+                FFMPEG_BIN,
+                "-y",
+                "-i",
+                video_path,
+                "-ss",
+                "00:00:00",
+                "-vframes",
+                "1",
+                "-vf",
+                scale_filter,
+                "-q:v",
+                "2",
+                output_path,
+            ]
+            run_ffmpeg(cmd2, capture_output=True, timeout=timeout)
 
-    return output_path
+        if not Path(output_path).exists() or Path(output_path).stat().st_size == 0:
+            raise RuntimeError(f"Thumbnail generation failed: {output_path}")
+
+        return output_path
+    except Exception:
+        # 失败时清理自己创建的临时文件
+        if _is_temp_output and output_path:
+            try:
+                Path(output_path).unlink(missing_ok=True)
+            except Exception:
+                pass
+        raise
 
 
 def _format_seek_time(seconds: float) -> str: