The Service Layer

Services orchestrate. They sit between your interfaces (CLI, API, bot) and your infrastructure (LLM providers, databases).

A service:

• Accepts interfaces (Protocol types), not concrete implementations — this is dependency injection
• Handles the "what" — call LLM → parse → save → return
• Is ignorant of how storage works or which model is called

Why care? The same ClassificationService powers the cohort's CLI, Telegram bot, and REST API without any changes. Swap MockProvider for ClaudeProvider at the call site — nothing else changes.

Task

Complete classify_and_save(description, amount):

1. Build a message asking the LLM to classify the description, requesting JSON {"category": "..."}
2. Call self.provider.complete(messages)
3. Parse the JSON to extract the category
4. Create and save an Expense via self.repo.add()
5. Return the saved expense