Analyze a YouTube channel and return a JSON object with: channel_id, channel_title, channel_url, sample_video_ids[], videos_analyzed (count of videos fetched from playlist), transcripts_available (count with actual caption content), topics[] (top keyword frequencies across all transcripts), topics_structured[] (per-video semantic analysis — each entry has video_id/theme/entities[]/tags[]), note (which analysis mode ran), and optional output_path (local artifact path). Requires YOUTUBE_API_KEY and APIFY_TOKEN; set GEMINI_API_KEY for topics_structured semantic analysis (falls back to keyword-only when absent). Supported channel inputs: @handle (e.g. @fireship), youtube.com/@handle URL, /channel/UC... URL, bare 24-char UCxxxxxx ID, or legacy /c/ and /user/ URLs.