[ISSUE-010] User Profile Schema & API

## Phase 2: User Management & Profiles (Week 3)

### Description
Implement User model with preferences and schedule fields
Create profile CRUD endpoints (GET, PUT /api/users/profile)
Create preferences endpoint (PUT /api/users/preferences)
Create schedule endpoint (PUT /api/users/schedule)
Add input validation with Zod

### Dependencies
- #005
- #008

### Can Work in Parallel With
- #011

Start by reviewing dependencies #005 and #008, then inspect the existing user and API structure. Implement the User model with preferences and schedule fields, profile CRUD endpoints, preferences and schedule update endpoints, and Zod validation. Done means all listed endpoints and fields are implemented and accept only valid input.

Written by the indexing model from the issue text.