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/api/routes/api/config.py

@@ -86,6 +86,12 @@ def put_config(
         logger.info("MQTT settings changed — triggering reconnect.")
         mqtt_manager.reconnect(refreshed_settings)
 
+    # Re-apply the DSMR subscription so enabling/disabling DSMR ingest (or changing
+    # its topic / sample interval) takes effect immediately, without an app restart.
+    # Done after any MQTT reconnect so it operates on the current client.
+    from app.services.dsmr_ingest import apply_dsmr_subscription
+    apply_dsmr_subscription(refreshed_settings)
+
     sections_raw = build_config_sections(db, refreshed_settings)
     return ConfigUpdateResponse(sections=_sections_from_raw(sections_raw))