feat: 七牛云存储支持（本地缓存/备份）

- StorageService 抽象层（store/read/exists）
- QiniuStorageService: 七牛云主存储（@ConditionalOnProperty）
- LocalStorageService: 本地文件存储（默认/@ConditionalOnMissingBean）
- CachedStorageService: 七牛主+本地缓存（写:七牛→本地, 读:本地→七牛）
- 配置: PAR_STORAGE_TYPE=qiniu + QINIU_AK/SK/BUCKET/REGION
- 不配七牛时自动降级到纯本地存储

par-api/src/main/resources/application.yml

@@ -46,6 +46,13 @@ mybatis-plus:
 par:
   static:
     path: ${PAR_STATIC_PATH:./data/static}
+  storage:
+    type: ${PAR_STORAGE_TYPE:local}   # qiniu / local
+    qiniu:
+      access-key: ${QINIU_AK:}
+      secret-key: ${QINIU_SK:}
+      bucket: ${QINIU_BUCKET:par-configs}
+      region: ${QINIU_REGION:z2}
   mediabot:
     psk: ${PAR_MEDIABOT_PSK:}
   admin: