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.

docs/design/m6-tibber-dynamic-energy.md

@@ -333,7 +333,7 @@ Phase D（API + 前端）
 - **Reviewer checklist**: profile 纯结构、无具体数值；策略用 Decimal；进出口分开（不相抵）；tibber 价匹配用 `starts_at ≤ t0` 最近一条；无 OOP 过度抽象（数据+函数，仿 M5 profiles.py）。
 
 ### M6-T04 — EnergyContract CRUD + 版本 + 校验（API）
-- **Status**: `todo` · **Depends**: M6-T01, M6-T03
+- **Status**: `done` · **Depends**: M6-T01, M6-T03
 - **Context**: 合同增删改、版本（改价加新版本、生效日期）、激活（一次一个）、按 profile 校验数值。
 - **Files**: `create app/api/routes/api/energy_contracts.py`、`app/schemas/energy_contract.py`、`app/services/contracts.py`；`modify app/main.py`（注册路由）；`create tests/test_api_energy_contracts.py`
 - **Steps**: