feat: multi-role support — comma-separated roles with checkbox UI

- Added validate_roles() and roles_contain() helpers for comma-separated roles
- Updated create_user/update_user to accept role combinations (e.g. 'admin,technician')
- Changed all exact role queries (='technician', IN('technician','admin')) to LIKE
- Replaced single <select> with checkboxes in add/edit user modals
- Added renderRoleBadges() to display multiple role badges per user
- Added checkbox-group/checkbox-label CSS styles
- Updated tech-photo-upload technician query to use LIKE
- Updated db reset auth check to use roles_contain()
- Set shawn's role to 'admin,technician'
- Cleaned up unused imports (hashlib, io, etc.)

Applies to canteen-admin-server, canteen-admin-server-dev, and tech-photo-upload

.gitignore

@@ -1,4 +1,7 @@
+.venv/
 __pycache__/
 *.pyc
-assets.db
-uploads/
+*.db
+*.db.real
+*.xlsx
+.env