M6-T06: extend MqttManager with subscribe + DSMR ingest

- mqtt.py: subscription registry, on_message dispatch (swallows handler
  errors), re-subscribe on (re)connect; publish/connect/disconnect/reconnect
  unchanged.
- app/services/dsmr_ingest.py: handle_message stores the full telegram frame
  (no allowlist; gas/phases/null preserved), 10s downsample by timestamp.second,
  source_id idempotency (select + IntegrityError rollback). Net-thread safe.
- main.py registers the DSMR subscription only when dsmr_ingest_enabled.

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

@@ -362,7 +362,7 @@ Phase D（API + 前端）
 - **Reviewer checklist**: token 不进日志；httpx 超时；upsert 幂等；时间存 UTC；无对 `tibber_price` 的破坏性批删。
 
 ### M6-T06 — MqttManager 扩订阅 + DSMR ingest
-- **Status**: `todo` · **Depends**: M6-T01, M6-T02
+- **Status**: `done` · **Depends**: M6-T01, M6-T02
 - **Context**: 给只发不收的 `MqttManager` 扩订阅；DSMR ingest 整帧 blob + 10s 降采样落库。
 - **Files**: `modify app/integrations/mqtt.py`（订阅注册 + on_message 分发）；`create app/services/dsmr_ingest.py`；`modify app/main.py`（启用时注册订阅）；`create tests/test_mqtt_subscribe.py`、`tests/test_dsmr_ingest.py`
 - **Steps**: