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.

internal/model/system_configs.go

@@ -1,4 +1,3 @@
-// Copyright 2025 linux.do
 // Copyright 2026 Arctel.net
 // SPDX-License-Identifier: Apache-2.0
 
@@ -49,6 +48,7 @@ const (
 	ConfigKeyDiskCacheLRUEnabled              = "disk_cache_lru_enabled"              // 是否启用 LRU 淘汰机制
 	ConfigKeyLoginSessionTTLHours             = "login_session_ttl_hours"             // 登录会话过期时间 (小时，0表示浏览器关闭后自动退出登录，-1表示永不过期)
 	ConfigKeyUpdateUpstreamRepository         = "update_upstream_repository"          // GitHub Actions Release 上游仓库
+	ConfigKeyStorageConfig                    = "storage_config"                      // 文件存储配置 (JSON)
 )
 
 const (
@@ -66,7 +66,7 @@ const (
 // SystemConfig 系统配置实体
 type SystemConfig struct {
 	Key         string    `json:"key" gorm:"primaryKey;size:64;not null"`
-	Value       string    `json:"value" gorm:"size:255;not null"`
+	Value       string    `json:"value" gorm:"type:text;not null"`
 	Type        string    `json:"type" gorm:"size:32;not null;default:'system'"`
 	Visibility  int       `json:"visibility" gorm:"not null;default:0"`
 	Description string    `json:"description" gorm:"size:255"`