refactor(task): replace init registration with bootstrap wiring

Introduce internal/bootstrap as the composition root with sync.Once
guards for task handler registration and push listener wiring. Replace
the single OnTaskCompleted global hook with multi-subscriber handlers
and remove init()-driven side effects from worker, admin task, and push.

internal/apps/admin/push/task_listener.go

@@ -15,8 +15,9 @@ import (
 	"github.com/Rain-kl/Wavelet/pkg/logger"
 )
 
-func init() {
-	task.OnTaskCompleted = handleTaskCompleted
+// RegisterTaskListeners subscribes push notification handlers to task completion events.
+func RegisterTaskListeners() {
+	task.OnTaskCompleted(handleTaskCompleted)
 }
 
 // handleTaskCompleted handles task completions and triggers appropriate push events.