feat: Add AI Video & Music Generation system

[xjanova]

Add comprehensive media generation infrastructure with placeholder implementation pending official API availability from Freepik and Suno AI.

Core Infrastructure:

• MediaModels.cs: Complete data models for video/music generation
• FFmpegService: Low-level FFmpeg operations (470 lines)
• VideoProcessor: High-level video processing (428 lines)
• AudioProcessor: High-level audio processing (508 lines)

Media Generation Workers:

• FreepikWorker: Freepik Pikaso AI video generation (placeholder mode)
• SunoAIWorker: Suno AI music generation (placeholder mode)
• MediaGenerationController: REST API with 6 endpoints

Key Design Decision:
Web Learning system investigated but NOT suitable because:

• Designed for posting automation (Facebook, IG, TikTok)
• Cannot extract generated content (video/audio URLs)
• WorkflowExecutionResult has no extraction mechanism

Current approach uses placeholder architecture until official APIs available.

Documentation:

• MediaGenerationDesign.md: Complete design rationale
• API_TESTING.md: Comprehensive testing guide
• IMPLEMENTATION_SUMMARY.md: Full implementation details

API Endpoints:

• POST /api/MediaGeneration/test/quick-video
• POST /api/MediaGeneration/test/quick-music
• POST /api/MediaGeneration/generate-video
• POST /api/MediaGeneration/generate-music
• POST /api/MediaGeneration/process-video
• GET /api/MediaGeneration/result/{taskId}

Build Status: ✅ Success (all projects compile)
Total New Code: ~4,000+ lines
New Files: 13 files
Modified Files: 6 files

🤖 Generated with Claude Code

Description

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

Related Issues

Screenshots (if applicable)