T-003: Create CoffeeBean and BrewSchedule domain models

[kavyashri-as]

Phase 2 — Domain Layer

Task: Create domain/models.py with CoffeeBean dataclass (id, name, quantity_kg, unit_cost, low_stock_threshold_kg, updated_at, computed is_low_stock), BrewSchedule dataclass, and BrewStatus enum (pending, in_progress, completed, cancelled).

Effort: 1 minute

Depends on: T-001

Acceptance Criteria: Both dataclasses instantiate with all TSD fields; is_low_stock returns correct boolean.