Backend:
- Add CommunityTariff entity (admin-managed max price + surcharge per community)
- Add UserTariff entity (user-agreed tariffs between metering points)
- Add TariffController (admin endpoints) and UserTariffController (user endpoints)
- Add TariffService with full validation (price <= max, ACTIVE state, membership)
- Add CommunityTariffChangedEvent and UserTariffChangedEvent
- Update NotificationListener to notify members on tariff changes
- Add 27 comprehensive tests for TariffService
Frontend:
- Add AdminTariffComponent (/dashboard/tariffs) for managing community tariffs
- Add UserTariffComponent (/dashboard/my-tariffs) for managing user tariffs
- Add routing and navigation entries
- Regenerate OpenAPI spec and frontend API client
Note: Old Tariff.java entity replaced by CommunityTariff + UserTariff
