feat: add HTTP API for programmatic file downloads

[na1oh]

Description

This PR adds an HTTP API for programmatic access to download files from Telegram messages.

Features

• POST /api/v1/download: Submit download tasks with Telegram message URL
• GET /api/v1/task/{id}: Query task status
• GET /health: Health check endpoint
• Token-based authentication: Optional authentication via Authorization header or query parameter
• CORS support: Cross-origin resource sharing enabled

Configuration

Add the following section to config.toml:

```toml
[api]
enable = true
host = "0.0.0.0"
port = 8080
token = "your-secret-token" # Optional
```

Usage Example

```bash

Submit a download task

curl -X POST http://localhost:8080/api/v1/download \
-H "Content-Type: application/json" \
-H "Authorization: Bearer your-token" \
-d '{
"url": "https://t.me/channel/123\",
"storage": "local"
}'

Check task status

curl http://localhost:8080/api/v1/task/abc123 \
-H "Authorization: Bearer your-token"
```

Implementation Details

• New pkg/api package with HTTP server implementation
• Configuration support in config/api.go
• Integration with existing core task queue
• Integration with existing tgutil for message parsing
• Comprehensive API documentation in docs/api.md

Related Issue

Closes #179