feat: Add Rust API Server with REST and GraphQL support

- Implement JWT authentication layer
- Add Engine Registry with 20+ conversion engines
- Implement Conversion Service with background job processing
- REST API endpoints for file conversion operations
- GraphQL API with queries and mutations
- Comprehensive error handling with conversion suggestions
- Integration tests for both REST and GraphQL APIs
- Complete API documentation

Features:
- REST API: /api/v1/* endpoints
- GraphQL API: /graphql endpoint with playground
- JWT Bearer token authentication
- Engine validation with alternative suggestions
- File download via API (not exposing file paths)
- Support for FFmpeg, ImageMagick, LibreOffice, Pandoc, and more

api-server/.gitignore

@@ -0,0 +1,25 @@
+# Environment variables
+.env
+.env.local
+.env.*.local
+
+# Build artifacts
+/target/
+
+# IDE
+.idea/
+.vscode/
+*.swp
+*.swo
+*~
+
+# OS files
+.DS_Store
+Thumbs.db
+
+# Test data
+/test_data/
+/data/
+
+# Logs
+*.log