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/.env.example

@@ -0,0 +1,19 @@
+# API Server 環境設定範例
+# 複製此檔案為 .env 並依需求修改
+
+# 伺服器設定
+API_HOST=0.0.0.0
+API_PORT=3001
+
+# JWT 設定（正式環境請務必更改！）
+JWT_SECRET=your-super-secret-jwt-key-change-in-production
+
+# 檔案儲存
+UPLOAD_DIR=./data/uploads
+OUTPUT_DIR=./data/output
+
+# 限制
+MAX_FILE_SIZE=104857600
+
+# JWT Token 過期時間（秒），用於驗證參考
+JWT_EXPIRATION_SECS=86400