Add MinerU document ingestion support

Normalize uploaded documents into structured markdown, add clearer parser
errors for missing dependencies, and cover the ingestion flow with
backend tests. This also replaces deprecated UTC timestamp helpers in
the touched backend paths so the knowledge pipeline stays warning-free.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/app/services/scheduler_service.py

@@ -32,9 +32,9 @@ async def daily_task_analysis():
     logger.info("[Scheduler] 开始执行每日任务分析...")
 
     async with async_session() as db:
-        from datetime import datetime, timedelta
+        from datetime import UTC, datetime, timedelta
 
-        yesterday = datetime.utcnow().date() - timedelta(days=1)
+        yesterday = datetime.now(UTC).date() - timedelta(days=1)
 
         # 统计昨日任务完成情况
         result = await db.execute(