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/config.py

@@ -63,6 +63,9 @@ class Settings(BaseSettings):
     dsmr_ingest_enabled: bool = False
     dsmr_mqtt_topic: str = "dsmr/json"
     dsmr_sample_interval_s: int = 10
+    # DSMR dual-tariff topic: publishes "1" (dal/off-peak) or "2" (normal/peak).
+    # Empty string = do not subscribe (tariff-aware pricing disabled).
+    dsmr_tariff_topic: str = "dsmr/meter-stats/electricity_tariff"
 
     # Tibber dynamic pricing credentials (M6; only used when active contract kind=tibber).
     tibber_api_token: str = ""