Title: [Users] Implement update user profile endpoint

Tags: backend, users, feature, good first issue

Description:
Allow authenticated users to update their display name and avatar.

Acceptance Criteria:

• PATCH /users/me — requires auth.
• Allows updating: username, avatarUrl.
• Validates username uniqueness and length (3–30 chars, alphanumeric + underscore).
• Returns 200 with updated user object.
• Returns 409 on duplicate username.

Files: src/routes/users.routes.ts, src/controllers/users.controller.ts