perf(config): add Otter RAM cache layer for system configs

Introduce pkg/cache/ram on top of existing Redis/DB config reads.
GetByKey now checks local RAM before Redis. Unified invalidation clears
RAM and Redis hash fields on admin writes, with pub/sub for multi-node
RAM eviction. Add tests and wire create/update/migrator write paths.

internal/db/redis.go

@@ -124,6 +124,17 @@ func HSetJSON[T any](ctx context.Context, hashKey, fieldKey string, data T) erro
 	return nil
 }
 
+// HDel removes one or more fields from a Redis Hash.
+func HDel(ctx context.Context, hashKey string, fieldKeys ...string) error {
+	if Redis == nil || len(fieldKeys) == 0 {
+		return nil
+	}
+	if err := Redis.HDel(ctx, PrefixedKey(hashKey), fieldKeys...).Err(); err != nil {
+		return fmt.Errorf(errRedisHashDeleteFailed, err)
+	}
+	return nil
+}
+
 // HGetJSON 从 Redis Hash 获取数据并反序列化为泛型类型
 // ctx: 上下文
 // hashKey: Redis Hash key