Add user-policy associations

[SakshiKekre]

This PR -

• Adds UserPolicy model to associate users with saved policies
• Adds CRUD endpoints at /user-policies/
• Adds 14 tests covering all endpoints

Endpoints

• POST /user-policies/ - create association
• GET /user-policies/?user_id=... - list by user (optional country_id filter)
• GET /user-policies/{id} - get by id
• PATCH /user-policies/{id} - update label
• DELETE /user-policies/{id} - delete association