Add API documentation in English and Chinese

Co-authored-by: krau <71133316+krau@users.noreply.github.com>

api/README.md

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+# API Module
+
+This module provides a RESTful HTTP API for programmatic file downloads from Telegram.
+
+## Features
+
+- **RESTful API endpoints** for creating, querying, and canceling download tasks
+- **Bearer token authentication** for API access control
+- **IP whitelist** support for additional security
+- **Webhook callbacks** for task completion notifications
+- **Task status tracking** (queued, running, completed, failed, canceled)
+- **Graceful shutdown** with proper cleanup
+
+## Usage
+
+See the full documentation at:
+- English: `/docs/content/en/usage/api.md`
+- Chinese: `/docs/content/zh/usage/api.md`
+
+## Architecture
+
+- `server.go` - HTTP server initialization and route registration
+- `handlers.go` - API endpoint handlers and business logic
+- `middleware.go` - Authentication and logging middleware
+
+The API integrates with the existing task queue system and uses the bot's Telegram client to fetch messages.