expose: buy/sell_price_now follow the live dual-tariff slot

Subscribe the separate DSMR tariff topic (dsmr/meter-stats/electricity_tariff,
1=dal/off-peak, 2=normal/peak), keep the latest slot in memory, and make the
manual-contract buy/sell_price_now sensors show the matching tariff's price
(tariff 1 -> dal, 2/unknown -> normal). Tibber (single 15-min price) unchanged.
Billing is untouched (register-split already handles tariffs correctly).

- new config dsmr_tariff_topic; subscribe managed by apply_dsmr_subscription
  (restart-free on config save). In-memory tariff is lock-guarded.
- import_cost_total / export_revenue_total cumulative getters unchanged.

tests/test_dsmr_subscription_apply.py

@@ -26,11 +26,18 @@ class _FakeMqtt:
         self.unsubscribe_calls.append(topic)
 
 
-def _settings(*, enabled: bool = True, topic: str = "dsmr/json", interval: int = 10):
+def _settings(
+    *,
+    enabled: bool = True,
+    topic: str = "dsmr/json",
+    interval: int = 10,
+    tariff_topic: str = "",
+):
     s = MagicMock()
     s.dsmr_ingest_enabled = enabled
     s.dsmr_mqtt_topic = topic
     s.dsmr_sample_interval_s = interval
+    s.dsmr_tariff_topic = tariff_topic
     return s
 
 
@@ -40,8 +47,9 @@ def fake_mqtt(monkeypatch):
     # apply_dsmr_subscription does `from app.integrations.mqtt import mqtt_manager`
     # at call time, so patching the module attribute is picked up.
     monkeypatch.setattr("app.integrations.mqtt.mqtt_manager", fake)
-    # Reset (and auto-restore) the module-level "currently subscribed topic".
+    # Reset (and auto-restore) the module-level "currently subscribed topics".
     monkeypatch.setattr(dsmr_ingest, "_current_dsmr_topic", None)
+    monkeypatch.setattr(dsmr_ingest, "_current_tariff_topic", None)
     return fake
 
 
@@ -90,3 +98,76 @@ def test_disabled_when_never_enabled_is_noop(fake_mqtt):
     assert fake_mqtt.subscribe_calls == []
     assert fake_mqtt.unsubscribe_calls == []
     assert dsmr_ingest._current_dsmr_topic is None
+
+
+# ---------------------------------------------------------------------------
+# Tariff topic subscription management
+# ---------------------------------------------------------------------------
+
+
+def test_enabled_with_tariff_topic_subscribes_both(fake_mqtt):
+    """When enabled and tariff_topic is non-empty, both topics must be subscribed."""
+    dsmr_ingest.apply_dsmr_subscription(
+        _settings(enabled=True, topic="dsmr/json", tariff_topic="dsmr/meter-stats/electricity_tariff")
+    )
+
+    subscribed_topics = [t for t, _ in fake_mqtt.subscribe_calls]
+    assert "dsmr/json" in subscribed_topics
+    assert "dsmr/meter-stats/electricity_tariff" in subscribed_topics
+    assert len(fake_mqtt.subscribe_calls) == 2
+    assert dsmr_ingest._current_tariff_topic == "dsmr/meter-stats/electricity_tariff"
+
+
+def test_enabled_with_empty_tariff_topic_subscribes_only_main(fake_mqtt):
+    """When tariff_topic is empty, only the main DSMR topic is subscribed."""
+    dsmr_ingest.apply_dsmr_subscription(
+        _settings(enabled=True, topic="dsmr/json", tariff_topic="")
+    )
+
+    assert len(fake_mqtt.subscribe_calls) == 1
+    assert fake_mqtt.subscribe_calls[0][0] == "dsmr/json"
+    assert dsmr_ingest._current_tariff_topic is None
+
+
+def test_disabled_after_tariff_subscription_unsubscribes_both(fake_mqtt):
+    """Disabling ingest must also unsubscribe the tariff topic."""
+    dsmr_ingest.apply_dsmr_subscription(
+        _settings(enabled=True, topic="dsmr/json", tariff_topic="dsmr/meter-stats/electricity_tariff")
+    )
+    dsmr_ingest.apply_dsmr_subscription(_settings(enabled=False))
+
+    assert "dsmr/json" in fake_mqtt.unsubscribe_calls
+    assert "dsmr/meter-stats/electricity_tariff" in fake_mqtt.unsubscribe_calls
+    assert dsmr_ingest._current_dsmr_topic is None
+    assert dsmr_ingest._current_tariff_topic is None
+
+
+def test_tariff_topic_change_resubscribes(fake_mqtt):
+    """Changing the tariff topic must unsubscribe the old one and subscribe the new one."""
+    old_tariff = "dsmr/meter-stats/electricity_tariff"
+    new_tariff = "meter/tariff"
+
+    dsmr_ingest.apply_dsmr_subscription(
+        _settings(enabled=True, topic="dsmr/json", tariff_topic=old_tariff)
+    )
+    dsmr_ingest.apply_dsmr_subscription(
+        _settings(enabled=True, topic="dsmr/json", tariff_topic=new_tariff)
+    )
+
+    assert old_tariff in fake_mqtt.unsubscribe_calls
+    subscribed_topics = [t for t, _ in fake_mqtt.subscribe_calls]
+    assert new_tariff in subscribed_topics
+    assert dsmr_ingest._current_tariff_topic == new_tariff
+
+
+def test_tariff_topic_cleared_unsubscribes(fake_mqtt):
+    """Setting tariff_topic to empty after it was subscribed must unsubscribe it."""
+    dsmr_ingest.apply_dsmr_subscription(
+        _settings(enabled=True, topic="dsmr/json", tariff_topic="dsmr/meter-stats/electricity_tariff")
+    )
+    dsmr_ingest.apply_dsmr_subscription(
+        _settings(enabled=True, topic="dsmr/json", tariff_topic="")
+    )
+
+    assert "dsmr/meter-stats/electricity_tariff" in fake_mqtt.unsubscribe_calls
+    assert dsmr_ingest._current_tariff_topic is None