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/services/config_page.py

@@ -123,6 +123,29 @@ CONFIG_FIELDS: tuple[ConfigField, ...] = (
         "HA Discovery Prefix",
     ),
     ConfigField("Modbus", "MODBUS_POLLING_ENABLED", "modbus_polling_enabled", "Modbus Polling Enabled", input_type="checkbox"),
+    ConfigField(
+        "DSMR",
+        "DSMR_INGEST_ENABLED",
+        "dsmr_ingest_enabled",
+        "DSMR Ingest Enabled",
+        input_type="checkbox",
+    ),
+    ConfigField("DSMR", "DSMR_MQTT_TOPIC", "dsmr_mqtt_topic", "DSMR MQTT Topic"),
+    ConfigField(
+        "DSMR",
+        "DSMR_SAMPLE_INTERVAL_S",
+        "dsmr_sample_interval_s",
+        "DSMR Sample Interval (s)",
+        input_type="number",
+    ),
+    ConfigField(
+        "Tibber",
+        "TIBBER_API_TOKEN",
+        "tibber_api_token",
+        "Tibber API Token",
+        secret=True,
+    ),
+    ConfigField("Tibber", "TIBBER_HOME_ID", "tibber_home_id", "Tibber Home ID"),
 )
 
 
@@ -322,4 +345,9 @@ def _settings_payload(settings: Settings) -> dict[str, Any]:
         "mqtt_tls_enabled": settings.mqtt_tls_enabled,
         "ha_discovery_enabled": settings.ha_discovery_enabled,
         "ha_discovery_prefix": settings.ha_discovery_prefix,
+        "dsmr_ingest_enabled": settings.dsmr_ingest_enabled,
+        "dsmr_mqtt_topic": settings.dsmr_mqtt_topic,
+        "dsmr_sample_interval_s": settings.dsmr_sample_interval_s,
+        "tibber_api_token": settings.tibber_api_token,
+        "tibber_home_id": settings.tibber_home_id,
     }