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.

app/services/config_page.py

@@ -138,6 +138,12 @@ CONFIG_FIELDS: tuple[ConfigField, ...] = (
         "DSMR Sample Interval (s)",
         input_type="number",
     ),
+    ConfigField(
+        "DSMR",
+        "DSMR_TARIFF_TOPIC",
+        "dsmr_tariff_topic",
+        "DSMR Tariff Topic",
+    ),
     ConfigField(
         "Tibber",
         "TIBBER_API_TOKEN",
@@ -348,6 +354,7 @@ def _settings_payload(settings: Settings) -> dict[str, Any]:
         "dsmr_ingest_enabled": settings.dsmr_ingest_enabled,
         "dsmr_mqtt_topic": settings.dsmr_mqtt_topic,
         "dsmr_sample_interval_s": settings.dsmr_sample_interval_s,
+        "dsmr_tariff_topic": settings.dsmr_tariff_topic,
         "tibber_api_token": settings.tibber_api_token,
         "tibber_home_id": settings.tibber_home_id,
     }