[ISSUE-025] RAG Service Implementation

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

### Description
Create RAG service class
Implement recipe retrieval based on user context
Build meal plan generation prompt
Integrate user preferences, inventory, and schedule
Add reasoning/explanation generation

### Dependencies
- #023
- #024

### Can Work in Parallel With
- #023
- #024

No files, tests, or entry points are named. Start by reviewing dependencies #023 and #024, then map the repository's AI infrastructure before scoping the RAG service, retrieval context, meal-planning prompt, preference and schedule integration, and explanation generation; done means all listed capabilities are integrated.

Written by the indexing model from the issue text.