[ISSUE-039] Leftovers Management

## Phase 6: Cost Tracking & External APIs (Weeks 10-11)

### Description
Implement Leftovers schema
Create leftovers tracking API
Add leftover logging in meal plan
Create leftovers screen in mobile app
Integrate leftovers into meal plan generation
Suggest recipes using leftovers

### Dependencies
- #026
- #030

### Can Work in Parallel With
- #038
- #040

Start by reviewing dependencies #026 and #030, then map the requested leftovers schema, tracking API, meal-plan logging and generation changes, mobile screen, and recipe suggestions across the project. Done means all six listed leftovers capabilities are integrated and usable.

Written by the indexing model from the issue text.