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
root
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