[Feature] Subtitle & Caption Generation - Auto-Generate and Edit Captions

[vzeman]

Summary

Implement subtitle and caption functionality including auto-generation from voice narration, a manual editor, and export options. Captions dramatically increase content accessibility and are required for many social media platforms.

Requirements

• Auto-generate captions from voice narration (transcribe the ElevenLabs output or input script)
• Manual subtitle editor: add, edit, delete, time-adjust subtitle entries
• Import SRT/VTT subtitle files
• Export SRT/VTT files
• Caption style editor: font, size, color, background, position, animation
• Preview captions overlaid on video player
• Burn-in captions during export (hard subtitles)
• Soft subtitles in exported video container
• Multi-language subtitle tracks
• Auto-translate captions using Claude API
• Word-level timing for karaoke-style highlighting

Acceptance Criteria

• Captions auto-generate from ElevenLabs narration script with accurate timing
• Manual editor allows adding, editing, and repositioning subtitle entries
• SRT/VTT import and export round-trips without data loss
• Burned-in captions appear correctly in the exported video
• Auto-translate produces subtitles in the target language using Claude

Related

Part of the ai-video-editor Flutter application. Depends on #67 (Voice & Audio Generation) and #72 (Final Video Export). Uses #75 API key management for Claude calls.