feat: endpoint to update budget_id for a given user GENAI-4011

[subpath]

Ticket: GENAI-4011

What's new:

• POST /user/{user_id}/budget — Update a user’s budget tier by service_type (e.g. ai, ai-dev); requires master_key and returns user_id, budget_id, service_type.

Tests ✅
Local QA ✅

1. Invalid service type will not work

2. Valid service typed will be assigned to the user