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_api_config.py

@@ -732,7 +732,7 @@ def test_put_config_mqtt_reconnect_uses_db_merged_settings(
 
 
 def test_get_config_includes_dsmr_section(client: TestClient) -> None:
-    """GET /api/config must include a DSMR section with expected fields."""
+    """GET /api/config must include a DSMR section with expected fields including DSMR_TARIFF_TOPIC."""
     _login(client)
 
     response = client.get("/api/config")
@@ -746,6 +746,9 @@ def test_get_config_includes_dsmr_section(client: TestClient) -> None:
     assert "DSMR_INGEST_ENABLED" in env_names
     assert "DSMR_MQTT_TOPIC" in env_names
     assert "DSMR_SAMPLE_INTERVAL_S" in env_names
+    assert "DSMR_TARIFF_TOPIC" in env_names, (
+        f"DSMR_TARIFF_TOPIC must be present in DSMR section; got {env_names}"
+    )
 
 
 def test_get_config_includes_tibber_section(client: TestClient) -> None:
@@ -903,6 +906,51 @@ def test_put_config_invalid_dsmr_sample_interval_returns_422_and_does_not_write(
     assert rows.get("DSMR_SAMPLE_INTERVAL_S") != "not-a-number"
 
 
+def test_put_config_dsmr_tariff_topic_persists_and_reflects_in_get(
+    client: TestClient, test_database_urls
+) -> None:
+    """DSMR_TARIFF_TOPIC must persist via PUT and be readable via GET /api/config."""
+    _login(client)
+
+    new_topic = "meter/tariff/slot"
+    payload = _full_config_payload({"DSMR_TARIFF_TOPIC": new_topic})
+    with patch("app.services.dsmr_ingest.apply_dsmr_subscription"):
+        response = client.put(
+            "/api/config",
+            json={"updates": payload},
+            headers={"X-CSRF-Token": "token"},
+        )
+    assert response.status_code == 200
+
+    # The updated value must appear in the GET response.
+    get_resp = client.get("/api/config")
+    body = get_resp.json()
+    dsmr_section = next(s for s in body["sections"] if s["name"] == "DSMR")
+    tariff_field = next(f for f in dsmr_section["fields"] if f["env_name"] == "DSMR_TARIFF_TOPIC")
+    assert tariff_field["value"] == new_topic, (
+        f"Expected DSMR_TARIFF_TOPIC to be {new_topic!r}, got {tariff_field['value']!r}"
+    )
+
+
+def test_put_config_dsmr_tariff_topic_in_settings_payload(client: TestClient) -> None:
+    """DSMR_TARIFF_TOPIC must appear in _settings_payload (GET /api/config returns it)."""
+    _login(client)
+
+    # The default value from Settings must appear in the DSMR section.
+    response = client.get("/api/config")
+    body = response.json()
+
+    dsmr_section = next(s for s in body["sections"] if s["name"] == "DSMR")
+    tariff_field = next(
+        (f for f in dsmr_section["fields"] if f["env_name"] == "DSMR_TARIFF_TOPIC"), None
+    )
+    assert tariff_field is not None, "DSMR_TARIFF_TOPIC must appear in DSMR config section"
+    # Default value should be the DSMR reader meter-stats topic.
+    assert tariff_field["value"] == "dsmr/meter-stats/electricity_tariff", (
+        f"Unexpected default for DSMR_TARIFF_TOPIC: {tariff_field['value']!r}"
+    )
+
+
 def test_get_config_tibber_api_token_value_masked_after_save(
     client: TestClient, test_database_urls
 ) -> None: