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.

tests/test_api_config.py

@@ -187,6 +187,28 @@ def test_put_config_with_csrf_header_updates_app_name(
     assert app_name_field["value"] == "Updated via API"
 
 
+def test_put_config_reapplies_dsmr_subscription(
+    client: TestClient, test_database_urls
+) -> None:
+    """Saving config must re-apply the DSMR subscription so enabling DSMR ingest
+    takes effect without an app restart (the route calls apply_dsmr_subscription
+    with the refreshed settings)."""
+    _login(client)
+
+    payload = _full_config_payload({"DSMR_INGEST_ENABLED": "true"})
+    with patch("app.services.dsmr_ingest.apply_dsmr_subscription") as spy:
+        response = client.put(
+            "/api/config",
+            json={"updates": payload},
+            headers={"X-CSRF-Token": "any-non-empty-value"},
+        )
+
+    assert response.status_code == 200
+    spy.assert_called_once()
+    applied_settings = spy.call_args.args[0]
+    assert applied_settings.dsmr_ingest_enabled is True
+
+
 def test_put_config_blank_secret_keeps_existing_value(
     client: TestClient, test_database_urls
 ) -> None: