M6-T08: add energy_cost expose provider + post-billing state publish

- expose.py: _energy_cost_provider yields buy/sell_price_now + import_cost_total
  /export_revenue_total (total_increasing, monetary), value_getters read from
  energy_cost_period; 2-element identifiers so the existing HA Discovery builder
  (which indexes identifiers[1]) works. Other providers/mechanism untouched.
- main.py: energy-cost job calls publish_states after computing periods (no-op
  when MQTT/Discovery off). Tests added.
Note: energy-cost sensors lack an availability heartbeat (ha_discovery mechanism
limitation, out of scope here).

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

@@ -393,7 +393,7 @@ Phase D（API + 前端）
 - **Reviewer checklist**: Decimal 算钱；寄存器差为"末−初"；进出口分开；周期边界 UTC 刻钟；快照不可变（重算才覆盖，且显式）；版本选择按 `effective_from≤t0<effective_to`；job 不崩。
 
 ### M6-T08 — energy_cost expose provider + state 发布
-- **Status**: `todo` · **Depends**: M6-T07
+- **Status**: `done` · **Depends**: M6-T07
 - **Context**: 复用 M5 expose/Discovery，发当前买/卖价 + 累计成本/收入。
 - **Files**: `modify app/integrations/expose.py`（`_energy_cost_provider` + 注册）；`modify app/services/energy_cost.py` 或 `app/main.py`（算完顺带 `publish_states`）；`create tests/test_energy_expose.py`
 - **Steps**: provider 产出 §3.7 实体（`buy_price_now`/`sell_price_now`/`import_cost_total`/`export_revenue_total`），累计项 `total_increasing`+`monetary`+currency；`value_getter` 从 `energy_cost_period` 取；计费 job 后 `publish_states`。