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/db/migrator/goose/sqlite/202606090001_initial_schema.sql

@@ -54,7 +54,7 @@ CREATE UNIQUE INDEX IF NOT EXISTS idx_external_accounts_source_external ON exter
 
 CREATE TABLE IF NOT EXISTS system_configs (
     key VARCHAR(64) PRIMARY KEY,
-    value VARCHAR(255) NOT NULL,
+    value TEXT NOT NULL,
     type VARCHAR(32) NOT NULL DEFAULT 'system',
     visibility INTEGER NOT NULL DEFAULT 0,
     description VARCHAR(255),
@@ -160,7 +160,8 @@ INSERT INTO system_configs (key, value, type, visibility, description, created_a
     ('email_register_verification_enabled', 'false', 'system', 1, '是否开启邮箱注册验证（true/false）', CURRENT_TIMESTAMP, CURRENT_TIMESTAMP),
     ('menu_display_config', '{}', 'system', 1, '目录显示配置（JSON 字符串，格式为 {url: enabled}）', CURRENT_TIMESTAMP, CURRENT_TIMESTAMP),
     ('search_engine_indexing_enabled', 'false', 'system', 1, '是否允许搜索引擎爬取/检索该站点（true/false）', CURRENT_TIMESTAMP, CURRENT_TIMESTAMP),
-    ('update_upstream_repository', 'Rain-kl/Wavelet', 'system', 0, 'GitHub Actions Release 上游仓库（owner/repo 或 GitHub 仓库地址）', CURRENT_TIMESTAMP, CURRENT_TIMESTAMP)
+    ('update_upstream_repository', 'Rain-kl/Wavelet', 'system', 0, 'GitHub Actions Release 上游仓库（owner/repo 或 GitHub 仓库地址）', CURRENT_TIMESTAMP, CURRENT_TIMESTAMP),
+    ('storage_config', '{"driver":"local","local":{"root":"."},"s3":{"region":"us-east-1"},"r2":{"region":"auto"},"minio":{"region":"us-east-1","path_style":true},"oss":{},"webdav":{}}', 'system', 0, '文件存储驱动及连接配置（JSON）', CURRENT_TIMESTAMP, CURRENT_TIMESTAMP)
 ON CONFLICT (key) DO NOTHING;
 
 INSERT INTO users (id, username, password, nickname, avatar_url, is_active, is_admin, last_login_at, created_at, updated_at)