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/bootstrap/bootstrap.go

@@ -0,0 +1,65 @@
+// Copyright 2026 Arctel.net
+// SPDX-License-Identifier: Apache-2.0
+
+// Package bootstrap wires cross-module integrations at the application composition root.
+// All registrations use sync.Once so entry points can call them safely without import-order side effects.
+package bootstrap
+
+import (
+	"sync"
+
+	admin_push "github.com/Rain-kl/Wavelet/internal/apps/admin/push"
+	"github.com/Rain-kl/Wavelet/internal/apps/admin/push/custom_events"
+	taskhandlers "github.com/Rain-kl/Wavelet/internal/task/handlers"
+)
+
+var (
+	registerTasksOnce            sync.Once
+	registerPushDomainEventsOnce sync.Once
+	registerTaskListenersOnce    sync.Once
+)
+
+// RegisterTasks registers all built-in task handlers and metadata.
+func RegisterTasks() {
+	registerTasksOnce.Do(func() {
+		taskhandlers.Register()
+	})
+}
+
+// RegisterPushDomainEvents wires push notification handlers for domain events.
+func RegisterPushDomainEvents() {
+	registerPushDomainEventsOnce.Do(func() {
+		custom_events.Register()
+	})
+}
+
+// RegisterTaskListeners wires operational listeners to task framework hooks.
+func RegisterTaskListeners() {
+	registerTaskListenersOnce.Do(func() {
+		admin_push.RegisterTaskListeners()
+	})
+}
+
+// RegisterAPI wires integrations required by the HTTP API process.
+func RegisterAPI() {
+	RegisterTasks()
+	RegisterPushDomainEvents()
+}
+
+// RegisterWorker wires integrations required by the task worker process.
+func RegisterWorker() {
+	RegisterTasks()
+	RegisterTaskListeners()
+}
+
+// RegisterScheduler wires integrations required by the task scheduler process.
+func RegisterScheduler() {
+	RegisterTasks()
+}
+
+// RegisterAll wires integrations for fused mode (API + Worker + Scheduler).
+func RegisterAll() {
+	RegisterTasks()
+	RegisterPushDomainEvents()
+	RegisterTaskListeners()
+}