[ISSUE-028] Schedule Management Backend

## Phase 5: AI Infrastructure & Meal Planning (Weeks 7-9)

### Description
Create schedule schema within User model
Implement time slot management
Add conflict detection logic
Create availability checking utilities

### Dependencies
- #010

### Can Work in Parallel With
- #026
- #027
- #029

Review dependency #010 and the existing User model first. The issue does not name files, entry points, or tests, so locate the model and related scheduling code before planning the schema, time-slot management, conflict detection, and availability utilities. Done should cover all four requested capabilities and their validation, but acceptance details are not specified.

Written by the indexing model from the issue text.