perf: webapp事件接口加分页、motion默认隔帧采样、新增20个单元测试

- /api/events 支持 limit/offset 分页参数，默认500上限2000
- MOTION_SAMPLE_STRIDE 默认从1改为2，长视频提速一倍
- 新增 tests/test_core.py：覆盖MD5、活动窗口合并、摔倒检测、活动标签合并、Ollama解析等20个纯函数测试

webapp/app.py

@@ -21,6 +21,8 @@ def index():
 def api_events():
     event_type = request.args.get("event_type")
     capture_date = request.args.get("date")  # YYYYMMDD
+    limit = min(int(request.args.get("limit", 500)), 2000)
+    offset = max(int(request.args.get("offset", 0)), 0)
 
     query = """
         SELECT e.id, e.video_id, e.event_type, e.label, e.start_offset_sec,
@@ -36,7 +38,8 @@ def api_events():
     if capture_date:
         query += " AND v.capture_date = ?"
         params.append(capture_date)
-    query += " ORDER BY e.timestamp_abs DESC, e.id DESC LIMIT 500"
+    query += " ORDER BY e.timestamp_abs DESC, e.id DESC LIMIT ? OFFSET ?"
+    params.extend([limit, offset])
 
     with get_connection() as conn:
         rows = conn.execute(query, params).fetchall()