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/src/main.rs

@@ -0,0 +1,44 @@
+//! ConvertX API Server - Main entry point
+//!
+//! A REST and GraphQL API server for file conversion operations.
+
+use std::net::SocketAddr;
+use tracing_subscriber::{layer::SubscriberExt, util::SubscriberInitExt};
+
+use convertx_api::{build_router, config::Config, AppState};
+
+#[tokio::main]
+async fn main() -> anyhow::Result<()> {
+    // Initialize logging
+    tracing_subscriber::registry()
+        .with(
+            tracing_subscriber::EnvFilter::try_from_default_env()
+                .unwrap_or_else(|_| "convertx_api=debug,tower_http=debug".into()),
+        )
+        .with(tracing_subscriber::fmt::layer())
+        .init();
+
+    // Load configuration
+    dotenvy::dotenv().ok();
+    let config = Config::from_env()?;
+
+    tracing::info!("Starting ConvertX API Server");
+    tracing::info!("REST API: http://{}:{}/api/v1", config.host, config.port);
+    tracing::info!("GraphQL Playground: http://{}:{}/graphql", config.host, config.port);
+
+    // Build application state
+    let state = AppState::new(config.clone());
+
+    // Build router
+    let app = build_router(state);
+
+    // Start server
+    let addr = SocketAddr::new(config.host.parse()?, config.port);
+    let listener = tokio::net::TcpListener::bind(addr).await?;
+    
+    tracing::info!("Server listening on {}", addr);
+    
+    axum::serve(listener, app).await?;
+
+    Ok(())
+}