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/cmd/worker-generate/main.go

@@ -53,7 +53,7 @@ func main() {
 		app.GenerationStreams,
 		generationCfg,
 		app.Config.LLM.MaxOutputTokens,
-	)
+	).WithCache(app.Cache)
 
 	promptRuleSvc := tenantapp.NewPromptRuleGenerationService(
 		app.DB,
@@ -63,7 +63,7 @@ func main() {
 		app.GenerationStreams,
 		generationCfg,
 		app.Config.LLM.MaxOutputTokens,
-	)
+	).WithCache(app.Cache)
 
 	ctx, cancel := context.WithCancel(context.Background())
 	defer cancel()