feat: add user management system with roles and permissions

- Introduce SQLite persistence via GORM (stored at <data>/.ushare.db)
- Add Role model with two built-in roles: admin (all perms) and user (upload only)
- Add three permissions: user_manage, upload, token_manage (reserved)
- Rewrite UserManager: DB-backed login with in-memory session tokens
- Auto-seed default roles and admin user on first startup
- Add AuthPermission middleware for fine-grained permission checks
- Add /api/uauth/me endpoint for current session info
- Add /api/admin/* CRUD routes for user and role management
- Add admin console page (/admin) with user table and role permissions view
- Show admin console link in share page for users with user_manage permission

🤖 Generated with [Qoder][https://qoder.com]

internal/pkg/db/client.go

@@ -46,6 +46,10 @@ func (c *Client) Session(ctxs ...context.Context) *gorm.DB {
 	return session
 }
 
+func (c *Client) Migrate(models ...interface{}) error {
+	return c.cli.AutoMigrate(models...)
+}
+
 func (c *Client) Close() {
 	d, _ := c.cli.DB()
 	d.Close()