feat(storage): add dynamic storage config and migration

Move storage backend configuration from startup YAML to system_config-backed runtime configuration. Add local, S3-compatible, R2, MinIO, OSS, and WebDAV backend support.

Add a storage migration async task using the existing task dispatch framework. Migration target config is carried in task payload, and maintenance mode is derived from task execution state.

Split upload file management and storage operations, add the admin storage configuration tab, and update migrations and Swagger docs.

docs/swagger.json

@@ -5594,8 +5594,7 @@
                     ]
                 },
                 "value": {
-                    "type": "string",
-                    "maxLength": 255
+                    "type": "string"
                 },
                 "visibility": {
                     "type": "integer",
@@ -5661,8 +5660,7 @@
                     "maxLength": 255
                 },
                 "value": {
-                    "type": "string",
-                    "maxLength": 255
+                    "type": "string"
                 },
                 "visibility": {
                     "type": "integer",