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/auth_service.py

@@ -1,4 +1,4 @@
-from datetime import datetime, timedelta
+from datetime import UTC, datetime, timedelta
 from passlib.context import CryptContext
 from jose import jwt, JWTError
 from app.config import settings
@@ -16,7 +16,7 @@ def get_password_hash(password: str) -> str:
 
 def create_access_token(data: dict, expires_delta: timedelta | None = None) -> str:
     to_encode = data.copy()
-    expire = datetime.utcnow() + (expires_delta or timedelta(minutes=settings.ACCESS_TOKEN_EXPIRE_MINUTES))
+    expire = datetime.now(UTC) + (expires_delta or timedelta(minutes=settings.ACCESS_TOKEN_EXPIRE_MINUTES))
     to_encode.update({"exp": expire})
     return jwt.encode(to_encode, settings.SECRET_KEY, algorithm=settings.ALGORITHM)