feat(mentor): add mentor module with CRUD and filtering

[Uyoxy]

Implements the Mentor module with full CRUD functionality, validation, and authorization rules as specified.

Features Implemented

• Mentor entity with profile fields and timestamps
• Create mentor profile (restricted to users flagged as mentors)
• List mentors with filtering by expertise and price range
• Get mentor by ID
• Update mentor profile (ownership enforced)
• Soft delete mentor profile
• Input validation and auth guards applied

Technical Details

• Expertise handled as array / relation (as per implementation choice)
• Query filters optimized for common search use cases
• Ownership checks enforced at service level
• All work done inside the drips folder as required

How to Test

1. Create a user flagged as mentor
2. Create mentor profile
3. Query mentors with expertise and price filters
4. Attempt update/delete as owner vs non-owner
5. Verify soft delete behavior

Closes #126