Add article summary feature with OpenAI integration

- Introduce 'summary' query parameter in /api/content/v1/parser endpoint
- Integrate OpenAI API for generating article summaries
- Add OpenAIKey field to Server struct and corresponding command-line
flag
- Update extractArticleEmulateReadability to handle summary requests
- Add generateSummary method using OpenAI's GPT-4o model (turns out to
be faster than even 4o mini)
- Add OpenAIClient interface and mock for testing
- Update README.md with new configuration options and API details

This feature allows users to request a summary of extracted articles
using OpenAI's GPT-4o model. To ensure secure usage, summary generation
requires a valid server token. The changes include comprehensive error
handling and test coverage for various scenarios, including token
validation and server misconfiguration.

Author: paskal