[ISSUE-020] Inventory Schema & API

## Phase 4: Inventory Management (Week 6)

### Description
Implement Inventory model
Create inventory endpoints (GET, PUT /api/inventory)
Create add ingredient endpoint (POST /api/inventory/ingredient)
Create remove ingredient endpoint (DELETE /api/inventory/ingredient/:name)
Add expiration date tracking

### Dependencies
- #005

### Can Work in Parallel With
- #021

Start by reviewing dependency #005 and the repository's existing API and model patterns. Implement the inventory model and the listed GET, PUT, POST ingredient, and DELETE ingredient endpoints, including expiration date tracking. Done means all specified routes and inventory behavior are present and consistent with the existing API.

Written by the indexing model from the issue text.