feat(cache): add read-through cache layer across all app services

Introduce a generic read-through caching infrastructure and wire it into
all major tenant app services to reduce database load on hot read paths.

Key changes:
- Add `DeletePrefix` to Cache interface with memory (prefix scan) and
  Redis (SCAN + DEL) implementations
- New `readthrough.go`: generic `LoadJSON` / `LoadJSONWithEmpty` helpers
  backed by singleflight to prevent cache stampedes; supports jittered TTL
- New `cache_support.go`: centralized cache key builders and invalidation
  helpers for all entities (workspace, brand, prompt rules, schedule tasks,
  articles)
- Wire optional cache into ArticleService, BrandService, WorkspaceService,
  PromptRuleService, ScheduleTaskService, TemplateService, MediaService,
  PromptGenerateService via `WithCache()` builder pattern
- ScheduleDispatchWorker invalidates schedule task cache after dispatching
- ArticleService gains a new `Detail` endpoint with empty-result caching
- Update cmd entrypoints and transport handlers to propagate cache

server/internal/tenant/transport/schedule_task_handler.go

@@ -16,7 +16,7 @@ type ScheduleTaskHandler struct {
 }
 
 func NewScheduleTaskHandler(a *bootstrap.App) *ScheduleTaskHandler {
-	return &ScheduleTaskHandler{svc: app.NewScheduleTaskService(a.DB, a.AuditLogs)}
+	return &ScheduleTaskHandler{svc: app.NewScheduleTaskService(a.DB, a.AuditLogs).WithCache(a.Cache)}
 }
 
 func (h *ScheduleTaskHandler) List(c *gin.Context) {