fix: align limitation with env (#1819)

### What problem does this PR solve?

try to process the large file will throw a File size exceeds error

### Type of change

- [x] Refactoring

Co-authored-by: Theta Wang (ncu) <chunshan.connect@gmail.com>

rag/settings.py

 except Exception as e:
     REDIS = {}
     pass
-DOC_MAXIMUM_SIZE = 128 * 1024 * 1024
+DOC_MAXIMUM_SIZE = int(os.environ.get("MAX_CONTENT_LENGTH", 128 * 1024 * 1024))
 
 # Logger
 LoggerFactory.set_directory(