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/integrations/mqtt.py

@@ -175,6 +175,27 @@ class MqttManager:
             except Exception:
                 logger.exception("MQTT subscribe error (topic=%s).", topic)
 
+    def unsubscribe(self, topic: str) -> None:
+        """Remove the handler for *topic* and unsubscribe from the broker.
+
+        Idempotent: unknown topics are ignored.  Used to apply config changes
+        without a restart (e.g. when DSMR ingest is turned off or its topic
+        changes).  If the client is connected, the broker unsubscribe is sent
+        immediately; either way the handler is removed from the registry so it
+        will not be re-subscribed on the next ``_on_connect``.
+        """
+        self._subscriptions.pop(topic, None)
+        with self._lock:
+            client = self._client
+            connected = self._connected
+
+        if client is not None and connected:
+            try:
+                client.unsubscribe(topic)
+                logger.debug("MQTT unsubscribed from topic=%s.", topic)
+            except Exception:
+                logger.exception("MQTT unsubscribe error (topic=%s).", topic)
+
     # ------------------------------------------------------------------
     # Internal helpers — must be called with self._lock held
     # ------------------------------------------------------------------