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

main_test.go

@@ -3,7 +3,6 @@ package main
 import (
 	"errors"
 	"strings"
-	"testing"
 
 	"github.com/navidrome/navidrome/plugins/pdk/go/host"
 	"github.com/navidrome/navidrome/plugins/pdk/go/pdk"
@@ -15,11 +14,6 @@ import (
 	. "github.com/onsi/gomega"
 )
 
-func TestDiscordPlugin(t *testing.T) {
-	RegisterFailHandler(Fail)
-	RunSpecs(t, "Discord Plugin Main Suite")
-}
-
 var _ = Describe("discordPlugin", func() {
 	var plugin discordPlugin
 
@@ -36,6 +30,8 @@ var _ = Describe("discordPlugin", func() {
 		host.SchedulerMock.Calls = nil
 		host.ArtworkMock.ExpectedCalls = nil
 		host.ArtworkMock.Calls = nil
+		host.SubsonicAPIMock.ExpectedCalls = nil
+		host.SubsonicAPIMock.Calls = nil
 	})
 
 	Describe("getConfig", func() {
@@ -122,6 +118,7 @@ var _ = Describe("discordPlugin", func() {
 		It("successfully sends now playing update", func() {
 			pdk.PDKMock.On("GetConfig", clientIDKey).Return("test-client-id", true)
 			pdk.PDKMock.On("GetConfig", usersKey).Return(`[{"username":"testuser","token":"test-token"}]`, true)
+			pdk.PDKMock.On("GetConfig", imageHostKey).Return("", false)
 
 			// Connect mocks (isConnected check via heartbeat)
 			host.CacheMock.On("GetInt", "discord.seq.testuser").Return(int64(0), false, errors.New("not found"))