T-004: Create InventoryService, BrewService, and domain exceptions

[kavyashri-as]

Phase 2 — Domain Layer

Task: Create domain/inventory.py (InventoryService) and domain/brewing.py (BrewService) with validation guards and status transition enforcement; create domain/exceptions.py with DuplicateBeanNameError, BeanNotFoundError, BrewNotFoundError, InvalidTransitionError.

Effort: 1 minute

Depends on: T-003

Acceptance Criteria: InvalidTransitionError raised for forbidden transitions; DuplicateBeanNameError raised on duplicate name.