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/task/routers.go

@@ -13,7 +13,6 @@ import ("fmt"
 	"github.com/Rain-kl/Wavelet/internal/apps/admin"
 	"github.com/Rain-kl/Wavelet/internal/model"
 	"github.com/Rain-kl/Wavelet/internal/task"
-	taskhandlers "github.com/Rain-kl/Wavelet/internal/task/handlers"
 	"github.com/Rain-kl/Wavelet/internal/task/scheduler"
 	"github.com/Rain-kl/Wavelet/pkg/logger"
 	"github.com/gin-gonic/gin"
@@ -21,10 +20,6 @@ import ("fmt"
 
 	"github.com/Rain-kl/Wavelet/internal/common/response")
 
-func init() {
-	taskhandlers.Register()
-}
-
 // ListTaskTypes 获取支持的任务类型列表
 // @Summary 获取支持的任务类型
 // @Description 返回系统支持的所有可调度任务类型列表，包括任务名称、描述、是否支持时间范围等元数据，需要管理员权限