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).

app/config.py

@@ -58,6 +58,16 @@ class Settings(BaseSettings):
     ha_discovery_enabled: bool = False
     ha_discovery_prefix: str = "homeassistant"
 
+    # DSMR smart-meter ingest via MQTT (M6; default off — opt-in).
+    # Subscribe to dsmr_mqtt_topic when dsmr_ingest_enabled=True.
+    dsmr_ingest_enabled: bool = False
+    dsmr_mqtt_topic: str = "dsmr/json"
+    dsmr_sample_interval_s: int = 10
+
+    # Tibber dynamic pricing credentials (M6; only used when active contract kind=tibber).
+    tibber_api_token: str = ""
+    tibber_home_id: str = ""
+
     model_config = SettingsConfigDict(
         env_file=".env",
         env_file_encoding="utf-8",