feat: 新增视频分析流水线、Web界面、孤儿清理工具；修复健壮性问题

- 新增 analysis/ 模块：MOG2活动检测、YOLO人物跟踪、MediaPipe摔倒检测、Ollama活动打标、缩略图抽取
- 新增 webapp/：Flask事件浏览界面，按需HEVC→H264转码缓存
- 新增 analyze.py：7阶段状态机流水线（scan→backfill→motion→person→fall→activity→thumbnail）
- 新增 cleanup_orphans.py：清理文件已删除的数据库孤儿记录
- 修复：motion/person/activity三阶段加单视频异常兜底，避免批次中断
- 修复：ollama_client加健康检查和3次重试，num_ctx从8192降到2048
- 修复：transcode_cache加fcntl文件锁，防止并发转码同一视频
- 修复：SQLite开WAL模式+busy_timeout，支持读写并发
- 修复：sync.py ffmpeg路径改为shutil.which查找，删除automate.py死代码PYTHON
- 修复：thumbnail失败加3次重试上限，避免无限重试损坏文件
- 新增：requirements.lock锁定版本、README文档
- 更新：.gitignore排除.playwright-mcp、截图、备份文件

analysis/thumbnail.py

@@ -0,0 +1,103 @@
+"""用 ffmpeg 在每条 events 记录的 start_offset_sec 处抽一帧，生成缩略图。
+
+供网页事件列表展示预览图，点击缩略图再跳转播放对应视频的完整时间点。
+
+失败重试上限：单个事件连续失败超过 MAX_ATTEMPTS 次后，在 extra_json 里标记
+thumbnail_failed，后续运行直接跳过，避免对损坏文件无限重试。
+"""
+import json
+import subprocess
+
+from analysis.config import PROJECT_ROOT, THUMBNAILS_DIR
+from analysis.db import get_events, get_video_by_id, update_event_thumbnail, update_event_extra
+
+MAX_ATTEMPTS = 3
+
+
+def _extract_frame(video_path, offset_sec, output_path):
+    # -ss 放在 -i 之后用精确seek：放在 -i 之前的快速seek在临近视频结尾处
+    # 会因缺少解码所需的参考帧而失败（HEVC "Could not find ref with POC"）。
+    # 录像多为几秒到几十秒的短片，精确seek的性能代价可忽略。
+    result = subprocess.run(
+        [
+            "ffmpeg", "-i", str(video_path), "-ss", str(offset_sec),
+            "-frames:v", "1", "-q:v", "3", "-y", str(output_path),
+        ],
+        capture_output=True, timeout=30,
+    )
+    return result.returncode == 0 and output_path.exists()
+
+
+def _get_attempts(event):
+    try:
+        extra = json.loads(event["extra_json"] or "{}")
+        return extra.get("thumbnail_attempts", 0)
+    except (json.JSONDecodeError, TypeError):
+        return 0
+
+
+def _increment_attempts(event):
+    try:
+        extra = json.loads(event["extra_json"] or "{}")
+    except (json.JSONDecodeError, TypeError):
+        extra = {}
+    extra["thumbnail_attempts"] = extra.get("thumbnail_attempts", 0) + 1
+    if extra["thumbnail_attempts"] >= MAX_ATTEMPTS:
+        extra["thumbnail_failed"] = True
+    update_event_extra(event["id"], json.dumps(extra, ensure_ascii=False))
+
+
+def process_all(limit=None):
+    THUMBNAILS_DIR.mkdir(exist_ok=True)
+    events = []
+    for e in get_events():
+        if e["thumbnail_path"]:
+            continue
+        try:
+            extra = json.loads(e["extra_json"] or "{}")
+            if extra.get("thumbnail_failed"):
+                continue
+        except (json.JSONDecodeError, TypeError):
+            pass
+        events.append(e)
+
+    if limit:
+        events = events[:limit]
+
+    video_cache = {}
+    processed = 0
+    failed = 0
+    for event in events:
+        video_id = event["video_id"]
+        if video_id not in video_cache:
+            video_cache[video_id] = get_video_by_id(video_id)
+        video = video_cache[video_id]
+        if video is None:
+            _increment_attempts(event)
+            failed += 1
+            continue
+
+        video_path = PROJECT_ROOT / video["file_path"]
+        if not video_path.exists():
+            _increment_attempts(event)
+            failed += 1
+            continue
+
+        output_path = THUMBNAILS_DIR / f"{event['id']}.jpg"
+        offset_sec = event["start_offset_sec"] or 0.0
+
+        if _extract_frame(video_path, offset_sec, output_path):
+            update_event_thumbnail(event["id"], str(output_path.relative_to(PROJECT_ROOT)))
+            processed += 1
+        else:
+            _increment_attempts(event)
+            failed += 1
+
+    return {"processed": processed, "failed": failed}
+
+
+if __name__ == "__main__":
+    print("===== 开始生成缩略图 =====")
+    result = process_all()
+    print(f"→ 生成 {result['processed']} 张缩略图，失败 {result['failed']} 条")
+    print("===== 生成完成 =====")