Refactor: Pydantic Configuration

[whoisjayd]

Problem Description

The current Config class manually parses environment variables and lacks robust validation. This leads to runtime errors if types are wrong.

Proposed Solution

Migrate configuration management to Pydantic.

Implementation Details

• Replace custom Config class with pydantic_settings.BaseSettings
• Define types and validators for API keys and numeric settings
• Load .env file automatically via Pydantic

Acceptance Criteria

• Config class uses Pydantic
• Validation errors are reported clearly at startup
• Existing functionality (env var override) remains intact

References

• src/yt_study/config.py