Fix: recent change made fetch_info unpicklable

[arabcoders]

• Introduced a new jsonl formatter to output structured JSON logs for file handlers.
• Updated API documentation (API.md) to reflect the new structured log format, including detailed log record fields and examples, and clarified the meaning of the fields and exception properties.
• Removed documentation for the now-deleted /api/thumbnail endpoint.
• Refactored yt-dlp logging: replaced the logger callback with a class-based _YTDLPLogger and introduced _LogCapture for capturing logs as strings. This improves testability and enables log target pickling.
• Changed the return type of extract_info_sync and fetch_info to return lists of strings for logs, rather than lists of dicts, and updated docstrings accordingly.
• Removed the /api/thumbnail endpoint and all related code.
• refactor: standardize JSONL structure.