Fase 2: read-only user role with per-category view toggles

Adds a third account type 'user' alongside super/admin: no create/edit/delete
rights on qr codes, view access to dynamic/static lists gated per-account by
two admin-controlled toggles (can_view_static, can_view_dynamic), and always
full visibility into the dashboard/reports regardless of those toggles.

- New columns can_view_static/can_view_dynamic on users (migrations/003)
- Users class + form_users.php: 'user' type option with the two toggles
- Access control: dynamic_qrcode.php/static_qrcode.php/bulk_action.php reject
  all mutations for type=user; dynamic_qrcodes.php/static_qrcodes.php enforce
  the view toggle and show all codes (no owner scoping, since 'user' owns none)
- Sidebar and list tables hide add/edit/delete/bulk UI for the read-only role
- index.php dashboard stats are unscoped for both 'super' and 'user'

src/authenticate.php

@@ -40,6 +40,8 @@ if ($_SERVER['REQUEST_METHOD'] === 'POST')
         $_SESSION['user_id'] = $row['id'];
 		$_SESSION['username'] = $row['username'];
 		$_SESSION['must_change_password'] = !empty($row['must_change_password']);
+		$_SESSION['can_view_static'] = !empty($row['can_view_static']);
+		$_SESSION['can_view_dynamic'] = !empty($row['can_view_dynamic']);
 		$_SESSION['last_activity'] = time();
 
 		audit_log('login_success');