Add user profile settings endpoints

[Akatenvictor]

What

Add user profile settings endpoints for updating display name, bio, avatar, and privacy preferences.

Why

Users need to customize their public profile. Currently there is no dedicated endpoint for managing profile settings beyond basic registration data.

Scope

• In: Profile update endpoint, avatar upload, bio field, profile privacy toggle
• Out: Profile verification badges, social media links, profile themes

Acceptance Criteria

• PUT /api/users/profile updates display name, bio, avatar
• Avatar upload via Pinata with thumbnail generation
• GET /api/users/profile returns current user's full profile
• GET /api/users/:id/public returns public profile (respects privacy settings)
• Privacy setting controls whether profile is publicly visible
• Input validation: name max 50 chars, bio max 500 chars
• Rate limiting on profile update endpoint

Technical Context

• UserController in src/controllers/ for profile handlers
• UserService in src/services/UserService for business logic
• User entity needs bio, avatarIpfsHash, isProfilePublic fields
• Database migration for new fields
• PinataService for avatar upload