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/routers/task.py

@@ -64,8 +64,8 @@ async def update_task(
         if field == "tags":
             setattr(task, field, json.dumps(value))
         elif field == "status" and value == TaskStatus.DONE:
-            from datetime import datetime
-            task.completed_at = datetime.utcnow()
+            from datetime import UTC, datetime
+            task.completed_at = datetime.now(UTC)
             setattr(task, field, value)
         else:
             setattr(task, field, value)