Add user service, data processing, auth utils, and API handlers

[vishnu-deepsource]

Adds core modules for the application:

• services/user_service.py: User account management with CRUD operations, authentication, caching
• services/data_processor.py: Data processing pipeline with CSV/XML parsing, statistics, batch processing
• utils/auth.py: Authentication utilities — password hashing, token generation/validation, authorization
• api/handler.py: API request handlers for search, file upload, exports, webhooks, and config management

Co-Authored-By: Warp agent@warp.dev