M6-T02: add DSMR + Tibber flat config fields

- Settings: dsmr_ingest_enabled, dsmr_mqtt_topic, dsmr_sample_interval_s,
  tibber_api_token (secret), tibber_home_id (all opt-in / off by default).
- CONFIG_FIELDS DSMR + Tibber sections; _settings_payload extended.
- .env.example examples; test_api_config coverage (sections, secret, 422).

.env.example

@@ -45,3 +45,15 @@ MQTT_TLS_ENABLED=false
 # Requires MQTT_ENABLED=true and a running MQTT broker.
 HA_DISCOVERY_ENABLED=false
 HA_DISCOVERY_PREFIX=homeassistant
+
+# Optional: DSMR smart-meter ingest via MQTT (M6; default off — opt-in).
+# Set DSMR_INGEST_ENABLED=true to subscribe to the DSMR MQTT topic and
+# store 10-second downsampled telegram frames in dsmr_reading.
+# DSMR_INGEST_ENABLED=false
+# DSMR_MQTT_TOPIC=dsmr/json
+# DSMR_SAMPLE_INTERVAL_S=10
+
+# Optional: Tibber dynamic pricing credentials (M6).
+# Only used when an active energy contract with kind=tibber is configured.
+# TIBBER_API_TOKEN=
+# TIBBER_HOME_ID=