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_dsmr_subscription_apply.py

@@ -0,0 +1,92 @@
+"""Tests for restart-free DSMR subscription management (apply_dsmr_subscription).
+
+These verify that toggling DSMR ingest / changing its topic / changing its sample
+interval via the config UI is reflected in the live MQTT subscription without an
+app restart.  A fake MQTT manager is injected so no real broker is touched.
+"""
+
+from __future__ import annotations
+
+from unittest.mock import MagicMock
+
+import pytest
+
+from app.services import dsmr_ingest
+
+
+class _FakeMqtt:
+    def __init__(self) -> None:
+        self.subscribe_calls: list[tuple[str, object]] = []
+        self.unsubscribe_calls: list[str] = []
+
+    def subscribe(self, topic: str, handler) -> None:
+        self.subscribe_calls.append((topic, handler))
+
+    def unsubscribe(self, topic: str) -> None:
+        self.unsubscribe_calls.append(topic)
+
+
+def _settings(*, enabled: bool = True, topic: str = "dsmr/json", interval: int = 10):
+    s = MagicMock()
+    s.dsmr_ingest_enabled = enabled
+    s.dsmr_mqtt_topic = topic
+    s.dsmr_sample_interval_s = interval
+    return s
+
+
+@pytest.fixture()
+def fake_mqtt(monkeypatch):
+    fake = _FakeMqtt()
+    # apply_dsmr_subscription does `from app.integrations.mqtt import mqtt_manager`
+    # at call time, so patching the module attribute is picked up.
+    monkeypatch.setattr("app.integrations.mqtt.mqtt_manager", fake)
+    # Reset (and auto-restore) the module-level "currently subscribed topic".
+    monkeypatch.setattr(dsmr_ingest, "_current_dsmr_topic", None)
+    return fake
+
+
+def test_enabled_subscribes_to_topic(fake_mqtt):
+    dsmr_ingest.apply_dsmr_subscription(_settings(enabled=True, topic="dsmr/json"))
+
+    assert len(fake_mqtt.subscribe_calls) == 1
+    assert fake_mqtt.subscribe_calls[0][0] == "dsmr/json"
+    assert fake_mqtt.unsubscribe_calls == []
+    assert dsmr_ingest._current_dsmr_topic == "dsmr/json"
+
+
+def test_disabled_after_enabled_unsubscribes(fake_mqtt):
+    dsmr_ingest.apply_dsmr_subscription(_settings(enabled=True, topic="dsmr/json"))
+    dsmr_ingest.apply_dsmr_subscription(_settings(enabled=False))
+
+    assert fake_mqtt.unsubscribe_calls == ["dsmr/json"]
+    assert dsmr_ingest._current_dsmr_topic is None
+
+
+def test_topic_change_unsubscribes_old_subscribes_new(fake_mqtt):
+    dsmr_ingest.apply_dsmr_subscription(_settings(topic="dsmr/json"))
+    dsmr_ingest.apply_dsmr_subscription(_settings(topic="meter/dsmr"))
+
+    assert fake_mqtt.unsubscribe_calls == ["dsmr/json"]
+    assert [t for t, _ in fake_mqtt.subscribe_calls] == ["dsmr/json", "meter/dsmr"]
+    assert dsmr_ingest._current_dsmr_topic == "meter/dsmr"
+
+
+def test_reapply_same_topic_resubscribes_fresh_handler(fake_mqtt):
+    # A changed sample interval must take effect — the handler is re-bound to a
+    # fresh settings snapshot, so re-applying the same topic re-subscribes.
+    dsmr_ingest.apply_dsmr_subscription(_settings(topic="dsmr/json", interval=10))
+    dsmr_ingest.apply_dsmr_subscription(_settings(topic="dsmr/json", interval=20))
+
+    assert len(fake_mqtt.subscribe_calls) == 2
+    assert fake_mqtt.unsubscribe_calls == []  # same topic, no churn
+    handler1 = fake_mqtt.subscribe_calls[0][1]
+    handler2 = fake_mqtt.subscribe_calls[1][1]
+    assert handler1 is not handler2  # fresh closure carrying the new settings
+
+
+def test_disabled_when_never_enabled_is_noop(fake_mqtt):
+    dsmr_ingest.apply_dsmr_subscription(_settings(enabled=False))
+
+    assert fake_mqtt.subscribe_calls == []
+    assert fake_mqtt.unsubscribe_calls == []
+    assert dsmr_ingest._current_dsmr_topic is None