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/energy/hooks.ts

@@ -115,12 +115,21 @@ export function useUpdateDevice() {
 // Mutation: delete device
 // ---------------------------------------------------------------------------
 
+export interface DeleteDeviceParams {
+  uuid: string
+  /** When true, perform a cascade delete (removes readings + expose toggles). */
+  cascade?: boolean
+}
+
 export function useDeleteDevice() {
   const qc = useQueryClient()
   return useMutation({
-    mutationFn: (uuid: string) =>
+    mutationFn: ({ uuid, cascade }: DeleteDeviceParams) =>
       apiClient.DELETE('/api/modbus/devices/{uuid}', {
-        params: { path: { uuid } },
+        params: {
+          path: { uuid },
+          ...(cascade ? { query: { cascade: true } } : {}),
+        },
       }),
     onSuccess: () => qc.invalidateQueries({ queryKey: ['modbus-devices'] }),
   })