M6-T04: add EnergyContract CRUD + versions + profile validation API
- app/services/contracts.py: create_contract, add_version (append-only,
auto-closes prior version), activate_contract (mutex), and
active_contract_version_at (half-open [from, to)) for the billing engine.
- app/schemas/energy_contract.py + routes/api/energy_contracts.py: 6 endpoints
(GET/POST contracts, GET/PATCH contracts/{id}, POST .../versions, GET profiles).
Values validated against pricing profiles (422 on mismatch); no DELETE.
- main.py registers router; OpenAPI re-exported; tests for CRUD/version/activate.
tliu93
4284bd40a7