feat(security): add configurable file upload size limits with optional splitting

- Add MAX_UPLOAD_SIZE config (default 1GB) to prevent resource exhaustion
- Add MAX_SINGLE_FILE_SIZE config for optional PDF file splitting
- Implement automatic PDF splitting when files exceed single file limit
- Update upload endpoint to use configured limits instead of hardcoded 500MB
- Add comprehensive tests for upload limits and file splitting
- Document configuration in ConfigurationGuide.md and SECURITY_AUDIT.md
- Reference SECURITY_AUDIT.md in error messages for user guidance

Co-authored-by: christianlouis <361235+christianlouis@users.noreply.github.com>

.env.demo

@@ -11,6 +11,16 @@ ALLOW_FILE_DELETE=true  # Allow deletion of file records
 PROCESSALL_THROTTLE_THRESHOLD=20  # Number of files above which throttling is applied (default: 20)
 PROCESSALL_THROTTLE_DELAY=3  # Delay in seconds between each task submission when throttling (default: 3)
 
+# **File Upload Size Limits** (Security - see SECURITY_AUDIT.md)
+# Maximum file upload size in bytes. Default: 1GB (1073741824 bytes)
+# Prevents resource exhaustion attacks. Adjust based on your server capacity.
+MAX_UPLOAD_SIZE=1073741824
+
+# Maximum size for a single file chunk in bytes (optional)
+# If set and a file exceeds this size, it will be split into smaller chunks for processing
+# Default: None (no splitting). Example: 104857600 for 100MB chunks
+# MAX_SINGLE_FILE_SIZE=104857600
+
 # **Authentication**
 AUTH_ENABLED=true
 # Generate a secure random string, for example: