feat(schedule): support KOL subscription targets and random time windows

Extend schedule tasks to dispatch KOL subscription prompts in addition
to prompt rules, add daily/weekly scheduling with fixed or random time
windows, and track dispatch outcomes (last run, status, consecutive
failures).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

server/internal/tenant/app/instant_task_service.go

@@ -3,7 +3,6 @@ package app
 import (
 	"context"
 	"encoding/json"
-	"fmt"
 	"time"
 
 	"github.com/jackc/pgx/v5/pgxpool"
@@ -201,8 +200,8 @@ func (s *InstantTaskService) List(ctx context.Context, params InstantTaskListPar
 			return nil, response.ErrInternal(50010, "scan_failed", err.Error())
 		}
 
-		item.ExecutionTime = stringPtrFromDBValue(executionAt)
-		item.CreatedAt = fmt.Sprintf("%v", createdAt)
+		item.ExecutionTime = timeStringPtrFromDBValue(executionAt)
+		item.CreatedAt = timeStringFromDBValue(createdAt)
 		item.Articles = parseInstantTaskArticleLinks(articlesJSON)
 		autoPublishPlatforms, err := loadScheduleAutoPublishPlatforms(ctx, s.pool, actor.TenantID, inputParamsJSON)
 		if err != nil {
@@ -228,14 +227,6 @@ func (s *InstantTaskService) List(ctx context.Context, params InstantTaskListPar
 	}, nil
 }
 
-func stringPtrFromDBValue(value interface{}) *string {
-	if value == nil {
-		return nil
-	}
-	text := fmt.Sprintf("%v", value)
-	return &text
-}
-
 func parseJSONMap(raw []byte) (map[string]interface{}, error) {
 	if len(raw) == 0 {
 		return map[string]interface{}{}, nil