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.

frontend/src/api/schema.d.ts

@@ -686,12 +686,23 @@ export interface paths {
          * Delete Device
          * @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
-         *     SELECT COUNT query — not by relying on SQLite's FK RESTRICT constraint,
-         *     which is not enforced at runtime in this project (no PRAGMA foreign_keys=ON).
+         *     **Cascade delete (cascade=true)**:
+         *     Permanently deletes the device together with all its readings and any
+         *     ``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.
          */
         delete: operations["delete_device_api_modbus_devices__uuid__delete"];
         options?: never;
@@ -3190,7 +3201,9 @@ export interface operations {
     };
     delete_device_api_modbus_devices__uuid__delete: {
         parameters: {
-            query?: never;
+            query?: {
+                cascade?: boolean;
+            };
             header?: {
                 "X-CSRF-Token"?: string | null;
             };