fix(plugins): silence plugin warnings and folder creation when plugins disabled (#4297)

* fix(plugins): silence repeated “Plugin not found” spam for inactive Spotify/Last.fm plugins

Navidrome was emitting a warning when the optional Spotify or
Last.fm agents weren’t enabled, filling the journal with entries like:

    level=warning msg="Plugin not found" capability=MetadataAgent name=spotify

Fixed by completely disable the plugin system when Plugins.Enabled = false.

Signed-off-by: Deluan <deluan@navidrome.org>

* style: update test description for clarity

Signed-off-by: Deluan <deluan@navidrome.org>

* fix: ensure plugin folder is created only if plugins are enabled

Signed-off-by: Deluan <deluan@navidrome.org>

---------

Signed-off-by: Deluan <deluan@navidrome.org>

plugins/host_scheduler.go

@@ -49,13 +49,13 @@ func (s SchedulerHostFunctions) CancelSchedule(ctx context.Context, req *schedul
 type schedulerService struct {
 	// Map of schedule IDs to their callback info
 	schedules  map[string]*ScheduledCallback
-	manager    *Manager
+	manager    *managerImpl
 	navidSched navidsched.Scheduler // Navidrome scheduler for recurring jobs
 	mu         sync.Mutex
 }
 
 // newSchedulerService creates a new schedulerService instance
-func newSchedulerService(manager *Manager) *schedulerService {
+func newSchedulerService(manager *managerImpl) *schedulerService {
 	return &schedulerService{
 		schedules:  make(map[string]*ScheduledCallback),
 		manager:    manager,