Add optional image hosting for non-public Navidrome instances

When Navidrome is behind a private network (e.g. Tailscale), Discord
cannot fetch artwork URLs. This adds optional integration with imgbb
(24h expiry, requires API key) and uguu.se (3h expiry, no key needed)
to upload cover art to a public host.

Closes #1

# Conflicts:
#	go.mod
#	go.sum

coverart.go

@@ -0,0 +1,180 @@
+package main
+
+import (
+	"encoding/base64"
+	"encoding/json"
+	"fmt"
+	"strings"
+
+	"github.com/navidrome/navidrome/plugins/pdk/go/host"
+	"github.com/navidrome/navidrome/plugins/pdk/go/pdk"
+)
+
+// Configuration keys for image hosting
+const (
+	imageHostKey   = "imagehost"
+	imgbbApiKeyKey = "imgbbapikey"
+)
+
+// imgbb API response
+type imgbbResponse struct {
+	Data struct {
+		DisplayURL string `json:"display_url"`
+	} `json:"data"`
+	Success bool `json:"success"`
+}
+
+// uguu.se API response
+type uguuResponse struct {
+	Success bool `json:"success"`
+	Files   []struct {
+		URL string `json:"url"`
+	} `json:"files"`
+}
+
+// getImageURL retrieves the track artwork URL, optionally uploading to a public image host.
+func getImageURL(username, trackID string) string {
+	imageHost, _ := pdk.GetConfig(imageHostKey)
+
+	switch imageHost {
+	case "imgbb":
+		return getImageViaImgbb(username, trackID)
+	case "uguu":
+		return getImageViaUguu(username, trackID)
+	default:
+		return getImageDirect(trackID)
+	}
+}
+
+// getImageDirect returns the artwork URL directly from Navidrome (current behavior).
+func getImageDirect(trackID string) string {
+	artworkURL, err := host.ArtworkGetTrackUrl(trackID, 300)
+	if err != nil {
+		pdk.Log(pdk.LogWarn, fmt.Sprintf("Failed to get artwork URL: %v", err))
+		return ""
+	}
+
+	// Don't use localhost URLs
+	if strings.HasPrefix(artworkURL, "http://localhost") {
+		return ""
+	}
+	return artworkURL
+}
+
+// uploadFunc uploads image data to an external host and returns the public URL.
+type uploadFunc func(contentType string, data []byte) (string, error)
+
+// getImageViaHost fetches artwork and uploads it using the provided upload function.
+func getImageViaHost(provider, username, trackID string, cacheTTL int64, upload uploadFunc) string {
+	// Check cache first
+	cacheKey := fmt.Sprintf("%s.artwork.%s", provider, trackID)
+	cachedURL, exists, err := host.CacheGetString(cacheKey)
+	if err == nil && exists {
+		pdk.Log(pdk.LogDebug, fmt.Sprintf("Cache hit for %s artwork: %s", provider, trackID))
+		return cachedURL
+	}
+
+	// Fetch artwork data from Navidrome
+	contentType, data, err := host.SubsonicAPICallRaw(fmt.Sprintf("/getCoverArt?u=%s&id=%s&size=300", username, trackID))
+	if err != nil {
+		pdk.Log(pdk.LogWarn, fmt.Sprintf("Failed to fetch artwork data: %v", err))
+		return ""
+	}
+
+	// Upload to external host
+	url, err := upload(contentType, data)
+	if err != nil {
+		pdk.Log(pdk.LogWarn, fmt.Sprintf("Failed to upload to %s: %v", provider, err))
+		return ""
+	}
+
+	_ = host.CacheSetString(cacheKey, url, cacheTTL)
+	return url
+}
+
+// getImageViaImgbb fetches artwork and uploads it to imgbb.
+func getImageViaImgbb(username, trackID string) string {
+	apiKey, ok := pdk.GetConfig(imgbbApiKeyKey)
+	if !ok || apiKey == "" {
+		pdk.Log(pdk.LogWarn, "imgbb image host selected but no API key configured")
+		return ""
+	}
+
+	return getImageViaHost("imgbb", username, trackID, 82800, func(_ string, data []byte) (string, error) {
+		return uploadToImgbb(apiKey, data)
+	})
+}
+
+// getImageViaUguu fetches artwork and uploads it to uguu.se.
+func getImageViaUguu(username, trackID string) string {
+	return getImageViaHost("uguu", username, trackID, 9000, func(contentType string, data []byte) (string, error) {
+		return uploadToUguu(data, contentType)
+	})
+}
+
+// uploadToImgbb uploads image data to imgbb and returns the display URL.
+func uploadToImgbb(apiKey string, imageData []byte) (string, error) {
+	encoded := base64.StdEncoding.EncodeToString(imageData)
+	body := fmt.Sprintf("key=%s&image=%s&expiration=86400", apiKey, encoded)
+
+	req := pdk.NewHTTPRequest(pdk.MethodPost, "https://api.imgbb.com/1/upload")
+	req.SetHeader("Content-Type", "application/x-www-form-urlencoded")
+	req.SetBody([]byte(body))
+
+	resp := req.Send()
+	if resp.Status() >= 400 {
+		return "", fmt.Errorf("imgbb upload failed: HTTP %d", resp.Status())
+	}
+
+	var result imgbbResponse
+	if err := json.Unmarshal(resp.Body(), &result); err != nil {
+		return "", fmt.Errorf("failed to parse imgbb response: %w", err)
+	}
+
+	if !result.Success {
+		return "", fmt.Errorf("imgbb upload was not successful")
+	}
+
+	if result.Data.DisplayURL == "" {
+		return "", fmt.Errorf("imgbb returned empty display URL")
+	}
+
+	return result.Data.DisplayURL, nil
+}
+
+// uploadToUguu uploads image data to uguu.se and returns the file URL.
+func uploadToUguu(imageData []byte, contentType string) (string, error) {
+	// Build multipart/form-data body manually (TinyGo-compatible)
+	boundary := "----NavidromeCoverArt"
+	var body []byte
+	body = append(body, []byte(fmt.Sprintf("--%s\r\n", boundary))...)
+	body = append(body, []byte(fmt.Sprintf("Content-Disposition: form-data; name=\"files[]\"; filename=\"cover.jpg\"\r\n"))...)
+	body = append(body, []byte(fmt.Sprintf("Content-Type: %s\r\n", contentType))...)
+	body = append(body, []byte("\r\n")...)
+	body = append(body, imageData...)
+	body = append(body, []byte(fmt.Sprintf("\r\n--%s--\r\n", boundary))...)
+
+	req := pdk.NewHTTPRequest(pdk.MethodPost, "https://uguu.se/upload")
+	req.SetHeader("Content-Type", fmt.Sprintf("multipart/form-data; boundary=%s", boundary))
+	req.SetBody(body)
+
+	resp := req.Send()
+	if resp.Status() >= 400 {
+		return "", fmt.Errorf("uguu.se upload failed: HTTP %d", resp.Status())
+	}
+
+	var result uguuResponse
+	if err := json.Unmarshal(resp.Body(), &result); err != nil {
+		return "", fmt.Errorf("failed to parse uguu.se response: %w", err)
+	}
+
+	if !result.Success || len(result.Files) == 0 {
+		return "", fmt.Errorf("uguu.se upload was not successful")
+	}
+
+	if result.Files[0].URL == "" {
+		return "", fmt.Errorf("uguu.se returned empty URL")
+	}
+
+	return result.Files[0].URL, nil
+}