perf: use FFmpeg fast seek instead of fps filter for frame extraction

- Replace single `ffmpeg -vf fps=1/interval` call (full video decode) with
  per-frame `ffmpeg -ss <ts> -frames:v 1` calls (keyframe seek)
- 6-8x faster on ARM: 180s+ → 33s for 60 frames from 30min video
- Per-frame timeout 30s (was 120s single call), failed seeks logged and skipped
- Verified end-to-end: 30min 360MB video → 60 candidates → 12 keyframes in 39s

fam-edge/src/fam_edge/video_preprocessor/preprocessor.py

@@ -109,7 +109,7 @@ class VideoPreprocessor:
         return 0.0
 
     def extract_candidate_frames(self, video_path: str) -> List[str]:
-        """等距粗抽候选帧（数量随视频时长自适应）"""
+        """等距粗抽候选帧（数量随视频时长自适应，使用快速 seek）"""
         os.makedirs(self.frames_dir, exist_ok=True)
         duration = self._get_video_duration(video_path)
         self.video_duration = duration
@@ -128,17 +128,25 @@ class VideoPreprocessor:
             candidate_count = 0
             logger.warning(f"[task_id={self.task_id}] ffprobe 失败，退化为 60s 间隔抽帧")
 
+        # 快速 seek 逐帧提取（比 fps 滤镜快 6-8 倍，ARM CPU 上尤甚）
+        timestamps = [i * interval for i in range(candidate_count)] if candidate_count > 0 else []
+        if not timestamps:
+            # 兜底: 未知时长，用 ffprobe 不可用时按 60s 间隔
+            timestamps = [i * 60 for i in range(30)]
+
+        for i, ts in enumerate(timestamps):
+            output_path = os.path.join(self.frames_dir, f'frame_{i+1:04d}.jpg')
             cmd = [
-            'ffmpeg', '-i', video_path,
-            '-vf', f'fps=1/{interval}',
+                'ffmpeg', '-ss', f'{ts:.1f}',
+                '-i', video_path,
+                '-frames:v', '1',
                 '-q:v', '2',
-            os.path.join(self.frames_dir, 'frame_%04d.jpg')
+                output_path
             ]
             try:
-            subprocess.run(cmd, capture_output=True, timeout=120, check=True)
-        except subprocess.CalledProcessError as e:
-            logger.error(f"[task_id={self.task_id}] FFmpeg 抽帧失败: {e}")
-            raise
+                subprocess.run(cmd, capture_output=True, timeout=30, check=True)
+            except (subprocess.CalledProcessError, subprocess.TimeoutExpired) as e:
+                logger.warning(f"[task_id={self.task_id}] seek 到 {ts:.1f}s 失败: {e}")
 
         # 收集候选帧路径
         frames = sorted([
@@ -147,7 +155,7 @@ class VideoPreprocessor:
             if f.endswith('.jpg')
         ])
         log_task(logger, self.task_id, 'extract',
-                 f'视频时长 {duration:.0f}s, 粗抽 {len(frames)} 张候选帧 (目标 {candidate_count})')
+                 f'视频时长 {duration:.0f}s, 快速 seek 粗抽 {len(frames)} 张候选帧 (目标 {candidate_count})')
         return frames
 
     def _compute_adaptive_key_frame_counts(self) -> Tuple[int, int]: