DSMR hardening: restart-free config + decouple from telegram id

Config hot-reload (no restart):
- MqttManager.unsubscribe(); apply_dsmr_subscription(settings) re-applies the
  DSMR subscription (subscribe/unsubscribe/topic-change, fresh snapshot so the
  sample interval also takes effect).
- Called from lifespan AND PUT /api/config, so toggling DSMR ingest in the UI
  takes effect immediately without restarting the app.

Decouple ingest from the DSMR telegram id (overflows / resets to zero):
- Migration 20260624_12: drop UNIQUE(source_id), make recorded_at UNIQUE.
- Idempotency now keys on recorded_at (telegram timestamp); the table's own
  autoincrement PK is the stable identity. source_id kept only as a reference.
- Regression test: colliding telegram ids no longer drop new data.

app/main.py

@@ -29,7 +29,7 @@ from app.config import get_settings
 from app.integrations.mqtt import mqtt_manager
 from app.services.auth import AuthBootstrapError, initialize_auth_schema
 from app.services.config_page import build_runtime_settings, seed_missing_config_from_bootstrap, sync_app_hostname_from_bootstrap
-from app.services.dsmr_ingest import handle_message as dsmr_handle_message
+from app.services.dsmr_ingest import apply_dsmr_subscription
 from app.services.public_ip import check_public_ipv4_and_notify
 from app.services.modbus_poll import poll_all_enabled_devices, BASE_POLL_TICK_SECONDS
 from app.services.ha_discovery import publish_discovery, publish_states
@@ -246,21 +246,10 @@ async def lifespan(_: FastAPI):
         _startup_session.close()
     mqtt_manager.connect(_startup_runtime_settings)
 
-    # DSMR ingest: subscribe to the configured MQTT topic when enabled.
-    # The handler captures a snapshot of the current runtime settings (for the
-    # dsmr_sample_interval_s value).  Runtime setting changes take effect after
-    # a server restart or an explicit reconnect — consistent with other MQTT
-    # configuration in this project.
-    if _startup_runtime_settings.dsmr_ingest_enabled:
-        _dsmr_topic = _startup_runtime_settings.dsmr_mqtt_topic
-        _dsmr_settings_snapshot = _startup_runtime_settings
-        mqtt_manager.subscribe(
-            _dsmr_topic,
-            lambda payload: dsmr_handle_message(payload, _dsmr_settings_snapshot),
-        )
-        logger.info("DSMR ingest enabled — subscribed to topic=%s.", _dsmr_topic)
-    else:
-        logger.debug("DSMR ingest disabled — not subscribing to DSMR topic.")
+    # DSMR ingest: subscribe to the configured MQTT topic when enabled.  The same
+    # applier is called from PUT /api/config, so toggling DSMR via the UI takes
+    # effect without an app restart.
+    apply_dsmr_subscription(_startup_runtime_settings)
 
     yield