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.

app/main.py

@@ -13,6 +13,7 @@ from sqlalchemy.orm import Session
 from app import models  # noqa: F401
 from app.api.routes.api.config import router as api_config_router
 from app.api.routes.api.data import router as api_data_router
+from app.api.routes.api.energy_contracts import router as api_energy_contracts_router
 from app.api.routes.api.expose import router as api_expose_router
 from app.api.routes.api.modbus import router as api_modbus_router
 from app.api.routes.api.session import router as api_session_router
@@ -188,6 +189,7 @@ def create_app() -> FastAPI:
     app.include_router(status.router)
     app.include_router(api_config_router)
     app.include_router(api_data_router)
+    app.include_router(api_energy_contracts_router)
     app.include_router(api_expose_router)
     app.include_router(api_modbus_router)
     app.include_router(api_session_router)