perf: pre-extract cover frames during render + FFmpeg fallback

- Add extract_cover_candidates() and extract_and_upload_cover_frames()
  to thumbnail_generator.py: extract 3 frames at 25%/50%/75% of video
- Add cover_candidates field to RenderAdapterResult
- Call frame extraction after thumbnail gen in render_adapter._do_render()
- Write cover_candidates to plan.config in edit_plan_generation worker
- Check pre-stored frames in generation_cover.py before calling AI service
- Replace MediaKit remote API with FFmpeg HTTP range-request in
  _call_ai_cover_service (no full video download needed)
- Fix CoverTemplateResponse ValidationError when config is None
- Add unit tests for all new code paths
- Update existing tests to match new FFmpeg-based behavior

apps/worker/video_processing/thumbnail_generator.py

@@ -155,3 +155,122 @@ def generate_and_upload_thumbnail(
                 Path(thumbnail_path).unlink(missing_ok=True)
             except Exception:
                 pass
+
+
+def extract_cover_candidates(
+    video_path: str,
+    num_frames: int = 3,
+    *,
+    width: int = 640,
+    timeout: int = 30,
+) -> list[dict]:
+    """在视频时长 25%/50%/75% 处各抽一帧，返回候选帧信息列表。
+
+    Args:
+        video_path: 视频文件路径
+        num_frames: 抽帧数量（默认 3）
+        width: 输出宽度
+        timeout: 单帧超时（秒）
+
+    Returns:
+        [{"local_path": "...", "frame_time": 5.0}, ...]
+    """
+    from video_processing.ffmpeg_utils import FFMPEG_BIN, probe_duration, run_ffmpeg
+
+    try:
+        duration = probe_duration(video_path)
+    except Exception:
+        duration = 0.0
+
+    if duration <= 0:
+        duration = 5.0  # fallback
+
+    # 计算抽帧时间点：25%, 50%, 75%
+    ratios = []
+    for i in range(1, num_frames + 1):
+        ratios.append(i / (num_frames + 1))
+
+    results = []
+    for idx, ratio in enumerate(ratios):
+        frame_time = max(0.5, duration * ratio)
+        tmp = tempfile.NamedTemporaryFile(suffix=".jpg", delete=False)
+        tmp.close()
+        output_path = tmp.name
+
+        try:
+            seek_str = _format_seek_time(frame_time)
+            scale_filter = f"scale={width}:-1:force_original_aspect_ratio=decrease,format=yuvj420p"
+            cmd = [
+                FFMPEG_BIN, "-y",
+                "-ss", seek_str,
+                "-i", video_path,
+                "-vframes", "1",
+                "-vf", scale_filter,
+                "-q:v", "2",
+                output_path,
+            ]
+            run_ffmpeg(cmd, capture_output=True, timeout=timeout)
+
+            if Path(output_path).exists() and Path(output_path).stat().st_size > 0:
+                results.append({
+                    "local_path": output_path,
+                    "frame_time": round(frame_time, 2),
+                })
+            else:
+                Path(output_path).unlink(missing_ok=True)
+        except Exception as e:
+            logger.warning("封面候选帧抽取失败 ratio=%.2f: %s", ratio, e)
+            Path(output_path).unlink(missing_ok=True)
+
+    return results
+
+
+def extract_and_upload_cover_frames(
+    video_path: str,
+    plan_id: str,
+    num_frames: int = 3,
+) -> list[dict]:
+    """抽取封面候选帧并上传到 OSS。
+
+    Args:
+        video_path: 本地视频路径
+        plan_id: 剪辑计划 ID（用于 OSS 路径）
+        num_frames: 抽帧数量
+
+    Returns:
+        [{"image_url": "https://...", "frame_time": 5.0, "storage_key": "covers/xxx/frame_0.jpg"}, ...]
+    """
+    candidates = extract_cover_candidates(video_path, num_frames=num_frames)
+    if not candidates:
+        logger.warning("封面候选帧抽取为空: plan_id=%s", plan_id)
+        return []
+
+    results = []
+    for idx, cand in enumerate(candidates):
+        local_path = cand["local_path"]
+        frame_time = cand["frame_time"]
+        storage_key = f"covers/{plan_id}/frame_{idx}.jpg"
+
+        try:
+            from video_processing.oss_helpers import upload_to_oss
+
+            url = upload_to_oss(local_path, storage_key)
+            if url:
+                results.append({
+                    "image_url": url,
+                    "frame_time": frame_time,
+                    "storage_key": storage_key,
+                })
+                logger.info(
+                    "封面候选帧上传成功: plan_id=%s idx=%d frame_time=%.2f",
+                    plan_id, idx, frame_time,
+                )
+        except Exception as e:
+            logger.warning("封面候选帧上传失败: plan_id=%s idx=%d error=%s", plan_id, idx, e)
+        finally:
+            try:
+                Path(local_path).unlink(missing_ok=True)
+            except Exception:
+                pass
+
+    return results