Refactor fastapi_app

[santyr]

Refactor and split the fast_api app into multiple files for better readability and maintainability; organize the code into several modules based on their functionality.

Suggested structure:

Main Application File (main.py): Contains the FastAPI app creation and route definitions.

Configuration and Environment Variables (config.py): Handles loading environment variables and configuration settings.

Models and Schemas (models.py): Contains Pydantic models for request and response schemas.

Utility Functions (utils.py): Includes various utility functions like convert_to_sats, create_invoice, etc.

Database Operations (database.py): Manages database operations, like storing and retrieving data.

External API Integrations (external_apis.py): Functions that interact with external APIs like YouTube API, payment API, etc.

Async Tasks (tasks.py): Asynchronous tasks such as triggering the feeder, handling payments, etc.

Logging (logging.py): Setup and configurations for logging.