M6-T07: add metering billing engine + period job + summary + recompute

- app/services/energy_cost.py: register_at (register values at a UTC boundary),
  compute_period (register deltas x active-version strategy, snapshot price +
  contract_version_id, immutable unless overwrite), compute_closed_periods
  (bounded lookback), recompute_range (explicit overwrite), summarize
  (sum net + standing fees month/30 - heffingskorting year/365).
- Missing tibber price -> skip; missing reading -> degraded (zeroed amounts,
  null version), consistently on both create and recompute paths.
- main.py: 1-minute energy-cost job (existing jobs/MQTT untouched). Tests added.

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

@@ -378,7 +378,7 @@ Phase D（API + 前端）
 - **Reviewer checklist**: on_message 网络线程内只做短事务、吞异常不崩连接；整帧存（含 gas/各相）；null 容错；`source_id` 幂等；10s 降采样正确。
 
 ### M6-T07 — 计费引擎 + 周期 job + 汇总 + 重算
-- **Status**: `todo` · **Depends**: M6-T01, M6-T03, M6-T04, M6-T05, M6-T06
+- **Status**: `done` · **Depends**: M6-T01, M6-T03, M6-T04, M6-T05, M6-T06
 - **Context**: 每 15min 用寄存器差 × active 合同 strategy 算计量电费（快照、不可变）；汇总加固定费 − 抵扣；周期 job + 重算。
 - **Files**: `create app/services/energy_cost.py`；`modify app/main.py`（周期 job）；`create tests/test_energy_cost.py`
 - **Steps**: