[ISSUE-026] Meal Plan Schema & API

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

### Description
Implement MealPlan model
Create meal plan generation endpoint (POST /api/meal-plans/generate)
Create get meal plan endpoint (GET /api/meal-plans?weekStartDate=)
Create update meal plan endpoint (PUT /api/meal-plans/:id)
Create meal status update endpoint (PUT /api/meal-plans/:id/meal-status)
Create feedback endpoint (POST /api/meal-plans/:id/feedback)

### Dependencies
- #005
- #025

### Can Work in Parallel With
- #027

Start by reviewing dependencies #005 and #025, then trace the existing API and model conventions in the repository. Define the MealPlan model and implement the five listed endpoints, including generation, retrieval, updates, meal status, and feedback; done means each route and schema is integrated and usable.

Written by the indexing model from the issue text.