feat: add download commands (yt, ig, tt, fb) and middleware support

- Add core Middleware support via `core.Default.Use()`
- Implement download command handlers for YouTube, Instagram, TikTok, and Facebook
- Add regex-based URL validation for all download commands
- Add API client for fetching download links
- Add helpers for uploading and sending media to WhatsApp

Co-Authored-By: Claude Opus 4.7 <[email protected]>

api/client.go

@@ -0,0 +1,119 @@
+package api
+
+import (
+	"encoding/json"
+	"fmt"
+	"io"
+	"net/http"
+	"net/url"
+	"os"
+	"strings"
+	"sync"
+	"time"
+
+	"github.com/gabriel-vasile/mimetype"
+)
+
+// Client is the shared HTTP client for the Mono REST API.
+type Client struct {
+	BaseURL string
+	APIKey  string
+}
+
+const MaxDownloadBytes int64 = 256 * 1024 * 1024
+
+var (
+	defaultClient *Client
+	defaultOnce   sync.Once
+	httpClient    = &http.Client{Timeout: 2 * time.Minute}
+)
+
+func initDefault() {
+	baseURL := os.Getenv("MONO_BASEURL")
+	if baseURL == "" {
+		baseURL = "https://mono.fdvky.me/api/v1"
+	}
+	defaultClient = &Client{
+		BaseURL: strings.TrimRight(baseURL, "/"),
+		APIKey:  os.Getenv("MONO_API_KEY"),
+	}
+}
+
+// Default returns the shared API client, initialized lazily so env vars
+// loaded via godotenv in main.init() are already available.
+func Default() *Client {
+	defaultOnce.Do(initDefault)
+	return defaultClient
+}
+
+// Get calls the API and decodes the JSON response into target.
+func (c *Client) Get(path string, params map[string]string, target any) error {
+	u, err := url.Parse(c.BaseURL + path)
+	if err != nil {
+		return fmt.Errorf("api url: %w", err)
+	}
+	q := u.Query()
+	for k, v := range params {
+		q.Set(k, v)
+	}
+	u.RawQuery = q.Encode()
+
+	req, err := http.NewRequest("GET", u.String(), nil)
+	if err != nil {
+		return fmt.Errorf("api request: %w", err)
+	}
+	if c.APIKey != "" {
+		req.Header.Set("X-API-Key", c.APIKey)
+	}
+
+	resp, err := httpClient.Do(req)
+	if err != nil {
+		return fmt.Errorf("api: %w", err)
+	}
+	defer resp.Body.Close()
+
+	if resp.StatusCode != http.StatusOK {
+		body, _ := io.ReadAll(resp.Body)
+		return fmt.Errorf("api %d: %s", resp.StatusCode, strings.TrimSpace(string(body)))
+	}
+
+	return json.NewDecoder(resp.Body).Decode(target)
+}
+
+// Download fetches a URL and returns the raw bytes and MIME type.
+func Download(rawURL string) ([]byte, string, error) {
+	req, err := http.NewRequest("GET", rawURL, nil)
+	if err != nil {
+		return nil, "", fmt.Errorf("download request: %w", err)
+	}
+
+	resp, err := httpClient.Do(req)
+	if err != nil {
+		return nil, "", fmt.Errorf("download: %w", err)
+	}
+	defer resp.Body.Close()
+
+	if resp.StatusCode < 200 || resp.StatusCode >= 300 {
+		body, _ := io.ReadAll(io.LimitReader(resp.Body, 4096))
+		return nil, "", fmt.Errorf("download %d: %s", resp.StatusCode, strings.TrimSpace(string(body)))
+	}
+	if resp.ContentLength > MaxDownloadBytes {
+		return nil, "", fmt.Errorf("download too large: %d bytes", resp.ContentLength)
+	}
+
+	mime := resp.Header.Get("Content-Type")
+
+	data, err := io.ReadAll(io.LimitReader(resp.Body, MaxDownloadBytes+1))
+	if err != nil {
+		return nil, "", fmt.Errorf("read: %w", err)
+	}
+	if int64(len(data)) > MaxDownloadBytes {
+		return nil, "", fmt.Errorf("download too large: over %d bytes", MaxDownloadBytes)
+	}
+
+	if mime == "" || mime == "application/octet-stream" {
+		mime = mimetype.Detect(data).String()
+	}
+
+	return data, mime, nil
+}