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.

openapi/openapi.yaml

@@ -1332,16 +1332,37 @@ paths:
       description: 'Delete a Modbus device.
 
 
+        **Default behaviour (cascade=false)**:
+
         Returns 409 Conflict if the device has any associated readings; use
 
         ``enabled=false`` to disable it instead.
 
 
-        **Application-layer safety**: the check is performed via an explicit
+        **Cascade delete (cascade=true)**:
 
-        SELECT COUNT query — not by relying on SQLite''s FK RESTRICT constraint,
+        Permanently deletes the device together with all its readings and any
 
-        which is not enforced at runtime in this project (no PRAGMA foreign_keys=ON).'
+        ``ExposedEntityToggle`` rows whose key matches ``modbus.<uuid>.*``.
+
+        Also makes a best-effort attempt to clear the device''s HA Discovery
+
+        config topics from MQTT (empty retained payload) before the DB rows
+
+        are removed.  MQTT failures are swallowed — the DB deletion proceeds
+
+        regardless.
+
+        Returns HTTP 200 with a ``ModbusDeleteResponse`` JSON body on success.
+
+
+        **Application-layer safety**: the 409 guard uses an explicit SELECT COUNT
+
+        query to return a friendly message.  FK RESTRICT is enforced at runtime
+
+        (the app sets ``PRAGMA foreign_keys=ON``), so the cascade path deletes
+
+        readings before the device.'
       operationId: delete_device_api_modbus_devices__uuid__delete
       parameters:
       - name: uuid
@@ -1350,6 +1371,13 @@ paths:
         schema:
           type: string
           title: Uuid
+      - name: cascade
+        in: query
+        required: false
+        schema:
+          type: boolean
+          default: false
+          title: Cascade
       - name: X-CSRF-Token
         in: header
         required: false