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/redis.go

@@ -31,3 +31,27 @@ func (c *redisCache) Set(ctx context.Context, key string, value []byte, ttl time
 func (c *redisCache) Delete(ctx context.Context, key string) error {
 	return c.client.Del(ctx, key).Err()
 }
+
+func (c *redisCache) DeletePrefix(ctx context.Context, prefix string) error {
+	if prefix == "" {
+		return nil
+	}
+
+	pattern := prefix + "*"
+	var cursor uint64
+	for {
+		keys, nextCursor, err := c.client.Scan(ctx, cursor, pattern, 100).Result()
+		if err != nil {
+			return err
+		}
+		if len(keys) > 0 {
+			if err := c.client.Del(ctx, keys...).Err(); err != nil {
+				return err
+			}
+		}
+		cursor = nextCursor
+		if cursor == 0 {
+			return nil
+		}
+	}
+}