Create guardian and guardian_ninja schemas #33
Description
Add support for guardians and their relationship with ninjas.
Details:
Add a Guardian schema, context, and seeding.
Add a guardian_id field to User.
Create a guardian_ninja schema to handle the many-to-many relationship between guardians and ninjas, with its usual context and seeding.
Ninjas are already implemented in PR #31.
Ensure associations are properly set up between Guardian and Ninja when seeding.