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/shared/cache/memory.go

@@ -58,6 +58,21 @@ func (c *memoryCache) Delete(_ context.Context, key string) error {
 	return nil
 }
 
+func (c *memoryCache) DeletePrefix(_ context.Context, prefix string) error {
+	if prefix == "" {
+		return nil
+	}
+
+	c.mu.Lock()
+	for key := range c.entries {
+		if len(key) >= len(prefix) && key[:len(prefix)] == prefix {
+			delete(c.entries, key)
+		}
+	}
+	c.mu.Unlock()
+	return nil
+}
+
 func (c *memoryCache) evictLoop() {
 	ticker := time.NewTicker(60 * time.Second)
 	defer ticker.Stop()