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/models/base.py

@@ -1,12 +1,16 @@
 import uuid
-from datetime import datetime
+from datetime import UTC, datetime
 from sqlalchemy import Column, String, DateTime
 from app.database import Base
 
 
+def utc_now() -> datetime:
+    return datetime.now(UTC)
+
+
 class BaseModel(Base):
     __abstract__ = True
 
     id = Column(String(36), primary_key=True, default=lambda: str(uuid.uuid4()))
-    created_at = Column(DateTime, default=datetime.utcnow, nullable=False)
-    updated_at = Column(DateTime, default=datetime.utcnow, onupdate=datetime.utcnow, nullable=False)
+    created_at = Column(DateTime, default=utc_now, nullable=False)
+    updated_at = Column(DateTime, default=utc_now, onupdate=utc_now, nullable=False)