modbus: add explicit cascade delete (device + readings + toggles + HA cleanup)

A disabled device with historical readings still cannot be deleted by default
(409 guard stays, to prevent accidental data loss). A new explicit opt-in lets
the user remove it for real:

- DELETE /api/modbus/devices/{uuid}?cascade=true removes the device's readings
  and exposed_entity_toggle rows, then the device, in one transaction (FK order:
  readings before device). Best-effort clears the HA discovery configs first
  (MQTT not connected -> no-op, never blocks the delete). Default (no cascade)
  unchanged. Returns 200 with deletion counts.
- Frontend: a 'Force Delete (all data)' button appears only after the 409, with
  a red irreversible warning; cascade is sent only on that explicit action.
- Also corrected a stale comment: FK RESTRICT IS enforced at runtime now.

app/services/ha_discovery.py

@@ -407,6 +407,74 @@ def publish_device_state(session: Session, device: Any) -> None:
             )
 
 
+def clear_device_discovery(session: Session, device_uuid: str) -> None:
+    """Best-effort: send empty retained payloads to all HA Discovery config topics
+    for the given device, effectively removing the device's entities from HA.
+
+    This must be called **before** the device rows are deleted from the DB, so
+    that ``build_catalog`` can still enumerate the device's entities.
+
+    Behaviour
+    ---------
+    - No-op if MQTT / HA Discovery is not enabled or the MQTT client is not
+      connected.
+    - All exceptions are caught internally; this function never raises.
+      The caller proceeds with the DB deletion regardless of MQTT outcome.
+
+    Parameters
+    ----------
+    session:
+        Active SQLAlchemy session (device must still exist in DB at call time).
+    device_uuid:
+        UUID string of the device being deleted.
+    """
+    from app.config import get_settings
+    settings = build_runtime_settings(session, get_settings())
+    if not _should_publish(settings):
+        logger.debug(
+            "clear_device_discovery: skipped (MQTT/Discovery not enabled or not connected)"
+        )
+        return
+
+    discovery_prefix = settings.ha_discovery_prefix
+    state_prefix = settings.ha_state_topic_prefix
+
+    try:
+        catalog = build_catalog(session)
+    except Exception:
+        logger.exception(
+            "clear_device_discovery: failed to build catalog for device uuid=%s; skipping HA cleanup",
+            device_uuid,
+        )
+        return
+
+    cleared = 0
+    for entry in catalog:
+        entity = entry.entity
+        # Only clear entities belonging to this device.
+        if entity.device.identifiers[1] != device_uuid:
+            continue
+        try:
+            topic, _config = build_discovery_payload(entity, discovery_prefix, state_prefix)
+            mqtt_manager.publish(topic, b"", retain=True)
+            cleared += 1
+            logger.debug(
+                "clear_device_discovery: cleared config topic for entity %r → %s",
+                entity.key,
+                topic,
+            )
+        except Exception:
+            logger.exception(
+                "clear_device_discovery: error clearing entity %r; continuing", entity.key
+            )
+
+    logger.info(
+        "clear_device_discovery: cleared %d HA discovery topic(s) for device uuid=%s",
+        cleared,
+        device_uuid,
+    )
+
+
 def publish_device_offline(session: Session, device_uuid: str) -> None:
     """Publish an "offline" availability payload for *device_uuid*.